Ayuh,.... There's a Few options,....
Simplest is a clear hose, fulla water....
A Sight Level, or better yet, a Lazer Level can be rented at most rental shops,...
Once ya set the corner pins, 'n establish the high point, 'n low point,....
Lotsa string-lines, 'n pins gives ya something to work from....
or a Good operator on a dozer could probably grade it from the seat...
String-lines are the Key though...
Simplest is a clear hose, fulla water....
A Sight Level, or better yet, a Lazer Level can be rented at most rental shops,...
Once ya set the corner pins, 'n establish the high point, 'n low point,....
Lotsa string-lines, 'n pins gives ya something to work from....
or a Good operator on a dozer could probably grade it from the seat...
String-lines are the Key though...