With a mitre saw or other saw, cut the PVC into the desired lengths. Depending on the shape of your 12" pot, you may need to experiment with this a little.
You will need two lengths of pipe, one slightly (1" or so) larger than the other. I ended up cutting mine to 8.5 and 9.5 inches. I also cut these at an angle so the ends would taper slightly to fit more snugly in the pot.
Make sure your shorter pipe fits in the bottom of the 12" pot (without touching the bottom) and make sure the longer pipe fits over (and perpendicular to) the shorter pipe without touching it.