Most sailboats have the cabin sole
parallel to the water line.
If yours is so, level the boat with the jacks and keel
Set up 4 sawhorses with vertical 2 x 4s clamped to the horses.
Position these such that you can run stiff battens, (i borrowed 20ft lengths of 1" x 3" aluminum
tubes from a glass shop), run between, and clamped to the 2 x 4s athwartships, with the batten tops touching the bottom edge of the chosen waterline.
Stretch end-weighted line fore and aft between the batten tops, and slide these lines in and out so that they progressive touch the hull
, --and mark.
Under the stern you can use a 4ft level from the line to the hull.
Connect the dots with a long flexible wood batten, 1/8" thick X 2" wide, with 2 helpers.
Very true results. Raise or lower the hull ends to adjust the W/Line.