The bunks or rollers that your boat rests on when on its trailer provide the support that keep your boat upright and steady while travelling. Adjust them with a wrench to fit under the strongest parts of your boat's hull. Your boat dealer, as a manufacturer's authorised representative, knows where these strong points on your boat's hull are located and, if you ask, will point out these reinforced areas of your boat's hull (see Reference 1).
Place a 4-foot spirit level on top of your boat's transom, where you hang your outboard motor. Two of your assistants will hold the boat steady, balancing on its keel while one of your three assistants should watch the level during the bunk adjustment process and alert you if the boat begins to lean to one side of the trailer or the other.
Loosen the nuts holding the bunks and rollers -- other than the rollers or bunks under the keel -- in place and allow the bunks or rollers to move away from the hull. Use your adjustable wrench.
Raise the outermost bunk (or roller) on the port (left) side of the boat to the hull, placing it snugly against the hull at a location where your boat dealer has pointed out as being reinforced. Have one of your assistants hold the bunk in place while you tighten the bolts that hold the roller or bunk in place. Repeat this process for the starboard (right) side, then for any remaining bunks or rollers other than the keel bunk/roller
Check the spirit level one more time to ensure that the boat is sitting squarely on the trailer. Check the bolts one more time.
Use a threadlocker on the threads of the bolts that hold bunks and rollers in place. Nothing is as frustrating as having to readjust the bunks or rollers at a boat landing because they've been jarred loose by a bumpy road into the landing, or settled while launching or recovering a boat.
If the boat begins to lean and is uncontrollable, step away from the boat. Your insurance can buy a new boat, but not a new you.