First step, have the pipes marked. This is critical since hitting a gas line would not make for a fun day.

Step two, mark off where the posts go.

The string helps here. Each post is supposed to (ideally) just touch the string. However, as we'll see later, people with large power tools seem to be unable to actually find the center of an X mark and reliably drill holes straight.

Setting a few posts. These holes were dug by hand using a clamshell post hole digger. It was a bit of work, but the poles are EXACTLY right. Each is between 24 and 26 inches deep. At about 15 minutes per hole, and a sore back after 4 of them...

Here's a closeup of an awesomely set post. The concrete is a little high, but it should be ok. The intent here is to make the water run off into the ground and not into the post's hole, which should help limit rot.