I used FOUR 10-foot pieces of 1/2" pvc I had lying around. You could do it with 3 pieces if you were more organized with the cuts. I ended up with 4 extra 3.5' PVC sections.
You will also need a bunch of t-connectors. You can buy packs of 10 t-connectors from Home Depot or a similar store for a few bucks.
I used 16'x10' foot tarp folded in half the long way (so it was 8x10). You want a relatively new tarp so there aren't any leaks in it. The tarp will still let seep water through but it seems to hold it well enough to play in for a few hours.
Plan to make your pool about 2-3 feet smaller than the width/height of your tarp, this gives you an extra foot of tarp on each side for attaching the tarp to the frame.
Example: For and 8x10 foot tarp, make a 6x8' frame (actually you might want to go about a foot small than this -- I did a 4x6 frame and that worked pretty well
For this project I'll assume you are making a 4x6' frame
Note that the sizing here is approximate. Your goal is to have the long sides have a split in the middle for a supporting leg, and the short sides to have a long section and then a short section connecting the side leg to the corner (see later steps to clarify this)
All of this is approximate, with some extra connectors and a pvc cutter, you can just shorten things or lengthen them by connecting two pieces as needed.
[NOTE, PVC is a terrible building product for the environment, so this is a better project for using up unused scrap pvc rather than buying it new...]
Green Building council report on PVC:
Article - http://www.inhabitat.com/2007/03/26/pvc-no-good/
Report (PDF) - https://www.usgbc.org/ShowFile.aspx?DocumentID=2379
PVC pipe summary is on page 9 of the pdf; General summary of impacts is on page 11]