Same way as you use any other computer ... make sure all the cables are in place, and throw the power switch.
As to the fine details of it, it depends what system you're using. I'm going to go thru a few things, depending on the likelihood of what you actually MEAN, derived from what you've WRITTEN :-)
Q1/ Using something like a 286 PC that's only got DOS installed on it.
A1a/ As a paperweight. Seriously. I had one of these given to me by a friend of the family about 5 years ago. I had some geeky fun messing about with it, upgrading the memory to a whopping 2.5mb, putting a colour (!) video card in, making it run programs it probably was never intended to... but it now lives in the back of the cupboard. The only part of the system still in use is the keyboard (with a manufacture date: 1987) because it's a beautiful, quality piece of work. Even if I have to daisy chain a pair of AT-PS2 and PS2-USB adaptors to make it work with a modern PC.
Or, if you HAVE to make it work (e.g. it's part of your job, it's got files in it your neighbour wants rescued), or you fancy your own geeky fun...
A1b/ (Option 1) Install MS Windows on it, for the love of Pete. Unless it's SERIOUSLY old (>18, 20 years), it'll be able to handle some flavour of recognisable Windows (3.1) and you can get stuff out of it/run it day-to-day that way. If it IS seriously old, you might be able to hunt out Windows 2.0, MSDOS Shell, or Digital Research GEM...
NOTE: The next answer only works if the PC has Microsoft DOS installed on it (MSDOS) or some close variant (e.g. PC DOS, DR DOS) - keep your eyes on the screen as it's powering up until all the activity finishes, and it should at some point say what system it's using.
If it's got something kooky like CP/M, IBM OS/2, Unix, Linux, QNX, BSD or Solaris installed, then that's out of the scope of what I can advise you on i'm afraid. Better to go webhunting or find a manual.
A1c/ If you really want to try it, or have no alternative (e.g. no installation priveleges... don't have any discs or enough memory / free hard disc space... don't have a hard disc or a graphics-capable video card...) Use it like any other PC that has DOS installed... any 80s, 90s, and early 2000s computer that's got a pre-M.E. version of windows will have a fully working copy you can practice with, and probably help files to boot, maybe even a DOS manual. Even ME, 2000 and XP have a pseudo-DOS "command line" window you can bring up.
In this case, yes ... you get a mostly black screen, where almost all the output comes in the form of blocky light grey text (or amber/orange, or green, if you have a anti-eyestrain tinted monochrome monitor). You command the computer with Unix/Linux-like instructions that you type it .... don't worry, it's not like you're having to program the thing. It's more like playing a text adventure game. All you have to do is know what you want to do, what state the computer's conceptually in (from reading what's already onscreen), what instruction you need to put in to take it from the current state to the one you want (or, to display the info you need), and then to read what it prints as the result. Easy enough. Working with a mouse in windows is quite literally just a simplified - and in many ways, less powerful - version of this. One thing to bear in mind is that you normally can run only one program at a time, and the environment in which you work is basically like being permanently stuck inside windows explorer / file manager until you choose a program to run.
Some basic ones to get you started. You don't need to capitalise them, but I have done so for clarity:
C:\>_ --- the "command prompt".. you don't type it, it appears automatically, and shows that the computer is ready to accept instructions (i.e. the last program or instruction has ended/completed), plus the disc drive you are working on and the folder you are in. e.g. D:\DOCUMENTS\>_
HELP - if this works, you're laughing. For the versions of DOS where this is installed, it will give you a full list of available commands, comprehensive details on what it does, why/how you would use it, and what option switches there are, all through a fairly easy to navigate text menu system.
DIR - Lists the contents of the current folder (or DIRectory). Type it with the "P" switch (as DIR /P) to break the display of long lists into screen-size chunks, and DIR /? to show a list of other switches, e.g. to view hidden files, sort by date or what have you.
CD (Change Directory) + a folder name - enters that folder.
CD .. - that's dot-dot, if you were unsure. Goes "back" one folder level.
CD \ - backslash... returns to the root folder / bottom level of the drive
A:, C:, D:, E:, etc (plus Enter) - swaps control to the named disc drive. You can also use B: (second floppy drive), which even works on single floppy systems as a virtual drive, if you want to copy things between floppies without using the hard disc (warning: involves a LOT of physical disc swapping & ejecting!)
COPY
- does exactly what it says on the tin, takes the stated file (or folder) and copies it to the folder (or drive, drive+folder) you specify. If I remember right, you can enter a few (only a few) source filenames at once, seperated by commas. Also, you can use wildcards - * to represent any number of missing characters (e.g. BOING.* for any filetype, BOI* for Boil, boing.doc, boite2.txt), ? to represent a single missing one (e.g. FILE??.p?c for file00.pic, file01.pic, file9z.pic, filech.pac) so there's less repetitive typing.
- runs that program. Will generally only work with .COM, .EXE, and .BAT files.
COM files are usually small things that work as "part" of DOS, take a few option switches, and print all their output to the immediate screen, often only using a few lines. e.g. Format, or the actual Command line program.
EXE files are the more familiar type of program, taking over the system and screen entirely and doing whatever they damn well please. e.g. Scandisc, FDISK, or any number of games.
(there are exceptions to this rule however - for example EDIT.com, a notepad work-alike, that runs fullscreen..)
BAT files are something rarely seen on modern machines, but work as a sort of mix between a shortcut and a macro or user-written program. It's basically a pre-written list of instructions that you set in motion by typing it's name (max of 8 characters, too - easy :) rather than having to enter the whole shebang every time, saving anything between one and a couple of thousand lines of typing depending on the complexity of the task... e.g. swapping to a certain drive and folder, starting a certain program with a particular set of options, then deleting a load of temporary files and printing a short message when it exits. It also saves a heck of a lot of time if the BATch file describes a large number of very simple, quick-running tasks, as the computer can read the instructions out of the file FAR quicker than anyone could ever type them.
More advanced ones you will hopefully be able to research for yourself - or not need to use, if you're just grabbing some files off the thing - include
MD (Make Directory, ie create a new folder),
RD (Remove Directory - but only when it's empty),
DEL (DELete!),
the ever popular DELTREE (DELete directory TREE - nukes an entire folder and any files/subfolders it contains)
and FORMAT (obvious!) commands.
Plus a useful wildcard to make these (and COPY) affect all files and folders inside the current folder (or whole drive, if you're at the root) - *.*
Alternative answers:
Q2/ Using an old non-PC computer such as an Amiga, Atari ST, Apple Mac, QX68000, Acorn Archimedes, etc, or an IBM PC with a non-Microsoft graphical system installed (e.g. GEM).
A2/ Just use it as you might a limited/archaic subset of windows - they have AppleOS, Amiga Workbench, Atari GEM, etc installed, which are all workalikes (and in some cases, obvious sources for microsoft plagiarism, as e.g. features in 1985 copies of GEM didn't show up on Wintel PCs until Windows 95!). Very easy once you adapt to certain minor differences, like what the close box looks like :).
Q3/ Using some other old computer... e.g. a Unix workstation, mainframe via a terminal, Sinclair, Commodore 64, MSX, Amstrad, BBC, Oric, Jupiter, Dragon, PET, Cambridge Z80, Acorn Electron, Atari 400/800 and so on for an endless list....
A3/ It's back to the typing commands, I'm afraid, but this time it's not so easy - you will have to do an internet search for information on the actual machine, or see if you can get your hands on a bona fide user manual for it... they'll almost all be different. Some of them will have enough similarities that you can use the same or nearly the same commands on them - any old Unix-based workstation or mainframe, for example, will work a bit like DOS, as will one that uses CP/M (though the differences are, well, different). Some home computers also used CP/M, which was sort of like a text-based Windows of the 1975-1985 era... very common (I think even the C64 works a bit like it?). The simplest machines, the Sinclairs etc al, will be completely unique in their operation, and will likely present just the option of either loading something ready-made from tape (either a text menu, or you typing the command yourself), or literally programming it yourself with a built-in BASIC or other easy-to-use but slow and low-powered language.
Secret option Q4/ Something else, that I haven't thought of!
A4/ Argh, I don't know, I've spent 15 minutes typing all this rubbish out.... add more details to your question or ask again more verbosely :-)
Secret option Q5/ An old games console....
A5/ ack... unless it's actually one of the small programmable home computers above, the sequence is: blow the dust out, put in the game cartridge and the joysticks (joypads? how passe, dear), and turn on the power... and play immediately :-)
Oh .... and if your computer doesn't have a hard disc... don't forget your boot floppy! (This goes for Amigas and certain Ataris, too - at least to make them boot *quicker*)