It's easy: download DosBox from this site, it's the newest version available at the moment. Start DosBox and type following at the prompt:
mount c c:/games/quest
c:
start the game with it's .exe file
where c:/games/quest is a directory of Space Quest, could be other of course.
When the game is too slow or too fast, you can adjust the speed with
CTRL+F11 (slower) or
CTRL+F12 (faster).
Try to find out more details for yourself, like editing
dosbox.conf file, that could be sometimes necessary if some games don't start properly. But if you have more questions, just ask.
