I need to create an A5 booklet (by folding A4 sheets), and the only tool at my disposal at the moment is Photoshop, since I haven't got InDesign on the machine that I'm working on right now.
What's the best way to set up a booklet in Photoshop?
For Photoshop, A4 or A5 in a landscape format, at minimum 300dpi/ppi (because you're printing it):
Preferences > Guides, grids and slices make gridlines at every 50 PERCENT. typically with subdivisions every 1 unit.
Now when you go
View > Show > Grid you will get a clear subdivision of your page. If you need it to be 3 fold, simply show place your lines every 33.33%.
After that it's simply just placing things where you want them, bearing in mind that the grid in the center is where your fold will be.