Use a carpenters level. Lay it on the floor and level the trailer side-to-side. Then use the tongue jack to level front-to-back. Your precision here will determine the accuracy of your end result. Now, place your levels in your selected places and center the bubbles. If you are attaching the levels with screws seal around the levels and screws with silicon seal or equivalent sealant.
It's not hard to do. Just take your time and be meticulous.
Chuck - Sparta, TN
2012 Jay Flight 22FB, 2 x Honda EU2000i
2013 GMC Yukon XL Denali AWD
EDUCATION is what you get when you read the fine print.....
EXPERIENCE is what you get when you don't.