Skimmer Help.

Building a pond in my yard. It is 10x15 and maybe 30inches at its deepest point. I had a pond previously and never had a skimmer on it. How tricky is it to get the skimmer right ? What is the optimal depth the skimmer mouth opening should be? And what is the margin for error, as in too low or too high? I live in Texas and the ground was so hard to dig with all the limestone in it.

Thanks.