Trenchers
The best way to do the trenching for any project is to
use a trencher. Trenchers come in a wide variety of
sizes, and the one that best suits you depends on how
deep you plan on burying your lines.
For black plastic line, you bury at a depth of 6"-8" and
PVC depths vary. Consult your supplier for the
appropriate depth. Another thing to consider: if you
plan on driving heavy equipment or large vehicles on
your lawn you'll want to bury you lines deeper so they
don't crack.
You'll be able to find trenchers at your local rental store
or often times the supplier of your irrigation parts will
have trenchers to rent. They'll advise you on what
trencher you need based upon how deep you're putting
down the lines.