If you have System 7 running on a relatively new machine (a Mac IIx or newer)
then there's a Control Panel called "Memory" which contains a setting called
Virtual Memory. This item won't be available at all if you have an old Mac
like a Plus or SE. Assuming it is, all you have to do is turn it on and set
up how much of your computer's hard disk you want to use for memory space,
and restart your computer.

The things to know are: (1) part of your hard disk is being used for memory
in place of RAM, so you'll have less hard disk space for storing files;
(2) since the hard disk is a lot slower than real RAM, your computer will
almost certainly work slower; and (3) not all programs are compatible, so
you might have to turn VM back off again.

Final note: I've had a report that some early versions of the Macintosh IIci
weren't compatible with virtual memory even though they allow you to turn it
on, and might cause a problem that would require that you re-install your
system software. This is unlikely but worth knowing before you try it, in
case you have files that you haven't backed up.