When you install a wood attic ladder in the ceiling, you will have to trim the legs so that when you unfold the ladder, the feet contact the floor at the right angle and make for a stiff set of stairs.
This isn’t hard to do, but it’s one of those things that pays to measure twice and cut once.
The bottom of the ladder will fold forward so that once the legs are cut to the correct length, the legs will be straight and rest on the floor. This will make a rigid ladder and give you confidence that it won’t collapse or move under your weight.
With the legs folded back, and the stairs extended, you want to take two measurements on each leg. You want to measure the distant from the back of the leg to the floor, and the distance from the front of the leg to the floor. You will do this for each leg and you may have four different measurements. It makes sense that both rear measurements and both front measurements should be the same, but this may not be the case if the floor is slightly sloped, etc. When you measure the distances, use something rigid. Don’t use a tape measure because it will flex and droop and will not be exact. Instead, use something like a 1”x2”, line it up along the leg and make a mark. Then transfer this distance onto the leg once you extend the stair back out. Do this for both the front and back of each leg.
What you will have made is a cut line on the leg. Mark a line between the two marks and cut along this line. Do this with each leg and the attic ladder should be cut perfectly.