You may find this task easier if you hold the string off the sagging floor joists by attaching blocks of the same thickness at opposite sides of the basement.
How to level a floor foundation.
Instead it may be an issue with joists and beams below your floor that require shoring up.
With a taut string water or laser level make a straight line from one side of the foundation to the other.
This video goes over some basic foundation and.
If you suspect that you re working with an off level foundation you can get it ready for installation quickly by using a floor leveling compound.
This video is for more experienced builders or for those who want an inside view of foundations and subfloors.
In some cases pouring floor leveling compound will fix the problem.
Your foundation may not be the problem.
For instance you may have a dining room floor that end to end is level but between those two points are various sags and dips.
These footings support wood beams carrying the floor structure and walls above.
Nail them to the underside of the joists as near the foundation as you can.