Guess it depends on the make/style/brand of trailer. I changed the bunks out on a tracker trailer I had, did all the measuring and inspecting and materials list while boat was in the water on a weekday. That way I had easy access to all areas of the trailer w/o crawling under the boat. I then built the new bunks to the same specs as old ones, took everything to the boat ramp again, launched the boat and wife drove around the cove while I removed the old and install the new bunks. This trailer had "L" shaped uprights for the bunks to mount to with a 1/4" lag bolt. It's not rocket science, and don't use PT wood.