The general procedure is: (1) level side-to-side by pulling the low wheel onto leveling blocks if needed, (2) block the wheels with chocks to keep it from rolling, (3) unhitch and pull the tow vehicle forward, (4) level front-to-back using the tongue jack, then drop a couple of turns below level in the front, (5) place stabilizers at the back and tighten, (6) raise the tongue jack back to level, (7) put the third stabilizer on the tongue near the coupler and tighten, and (8) back off slightly on the tongue jack to transfer weight
to the stabilizer.
Sounds complicated, but it will become natural quickly. Notice that the tongue jack is doing all the lifting, not the stabilizers, and that most of the weight
remains on the axle
and wheels. Stabilizers should always be placed on the main frame members of the trailer. I think on a Boler
the curved bumper is part of the main frame, so you could put them there, or a little forward under the cabin. In soft dirt or sand it may be necessary to put blocks under the stabilizers to keep them from sinking in.