Dig down to the pipe, make sure there are no pipes or wires below that, then take a 4' piece of 1" sch 40 pvc and a rubber mallet put one end of the pvc where you want the pop-up to be and pound the pipe into the ground as deep as you need, pull the pipe back out and you have your pop-up hole. Dirt can be removed from pipe by stout screwdriver or beating on a fence post.