This article is from the Miscellaneous Macintosh FAQ, by Elliotte Rusty Harold firstname.lastname@example.org with numerous contributions by others.
You need to upgrade to System 7.5 and install QuickDraw GX. This
requires a Mac with at least five megabytes of RAM. Eight megabytes
is a more realistic figure. However the background printing in
QuickDraw GX is quite stable and does not significantly decrease
the speed of foreground applications.
The above-mentioned MacPalette II provides background printing on an
ImageWriter under System 7 and a 68020 or better CPU. These are
fully commercial products. There are NO freeware, shareware, or
other ftpable solutions that work under System 7 so get out your
credit cards. At $45 for MacPalette but less than $300 for a
vastly superior DeskWriter or StyleWriter II you may want to forgo
the software and buy a better printer instead.
If you're still using System 6 and have no plans to move to
System 7, there is a shareware product called MultiSpool from Italy;
but it is not System 7 compatible and prints only under MultiFinder.