I tried the line level method (you can see 2 strings in the photos) the lines just sagged even with the tiny light weight levels. So this is my method--newbie at building. I will even each side and several spots through the middle, then check as I get the base installed. Turns out I need about another ton of bags of dirt. ;( Took my 10 foot board and raised the low end until it showed level. I now know about how much dirt I need to haul and will check all sides and the middles later.