Measure the angle between the basement floor and the riser of the sump pump. Adjust the riser if necessary with a wrench so that it forms a 90-degree angle with the floor.
If your sump pit cover has a slot with an open end, slip the sump pit cover over the riser. Seat the sump pit cover flush to the basement floor. Ensure that the float which turns the sump pump off and on has sufficient room to slide up and down the cover.
Measure the diameter of the riser with a measuring tape if your sump pit cover does not have a slot. Measure the diameter of the cover, and mark its midpoint with a pencil.
Set a drafting compass to the diameter of the riser plus about 1/4 of an inch. Place the point of the compass on the sump pit cover's midpoint. Circumscribe a circle on the sump pit cover with the compass pencil.
Fit your drill with a boring bit. Drill out the circumscribed circle on the cover. Test the cover's fit by sliding it over the pump's riser, and set it into place flush with the basement floor. If the cover does not slide easily over the riser, remove it and smooth down its center circle with a file.