From: Making booklets with psbook and psnup
Step 1: Start off with a Postscript file containing four pages. For an A5 size booklet, the pages need to be A4 size.
Step 3: Put pages one and four together to make the front and back. Put pages two and three together to make the centre spread.
Step 4: Place two-up A4 paper and rotate anticlockwise to landscape orientation.
Step 5: Print on both sides of one sheet of A4 paper.
Step 6: Turn clockwise.
Step 7: Fold page 4 behind page 1.

Hey presto, you have a four page, two leaf booklet.

Booklet printing the easy way:

Luckily, the magical psbook and psnup commands make this transformation easy. To make a A5 booklet out of a A4 document:

1. Create a Postscript file from your document.
2. Arrange “print.ps” into book order:
   $ psbook print.ps out.ps
3. Put the pages two to a page in landscape orientation:
   $ psnup -pa4 -2 out.ps > out2up.ps
4. The result is the output file “out2up.ps”. You can then view the output using gv or kghostview, and send it to your printer using lpr -o dupley=NoTumble.
5. If you have Ghostscript installed then you can convert the result into Adobe Portable Document Format (PDF):
   $ ps2pdf out2up.ps