Just chose Side fold booklet in Page Setup, and PagePlus will do the imposition of A5 on A4 landscape for you. Facing pages and dual master pages are the usual options too. If you publish to PDF, you choose "impose pages."
Click on the page number in the hintline to go to the master page to create frames for headers/footers, and insert page number codes.
In Format, page number format you can set the page to -1 to start the booklet with page 1 on physical page 3 to allow for the cover and inside front cover.
Bhikkhu's method is perfect because it means you can publish the PDF as either imposed pages (which will result in printer's pairs) or to publish the pages individually. Different printers have different requirements for this, so having the option is very useful.
Bhikkhu - do you have any tips on dealing with page creep? This may be an issue with a 32 page booklet.
The way to allow for page creep is to manually shift the frames using nudge. Calculate the total creep you require for 8 sheets, and adjust the nudge value to suit. Lock the frames after nudging them by ever increasing amounts from the centre-fold out, and save the document as a template for future use.