If you just begin laying wood floor planks along one wall of an old room and work your way across, you may get to the other end and find that the boards for the last course have to be cut practically triangular to fit, because the room is out of square. That's why it's important to carefully lay out and adjust your floor plan before you nail down the first plank. You have to leave gaps at the edges of the floor anyway (to accommodate wood expansion), so use those gaps to split the difference in an out-of-square room.
Remove all the floor trim in the room, using your hammer and prybar. Get the floor clean and dry.
Roll flooring paper over the floor, starting at the longest wall and laying it in slightly overlapping courses. Staple the paper down every few feet.
Measure 1/2 inch out from one end of the wall where you want to start (generally, the longest wall in the room) and mark it on the flooring paper with a pencil. Do the same thing at the other end of the wall.
Hold your chalk snapline between the two marks. Snap it, leaving a straight line on the flooring paper 1/2 inch out from the wall. Do the same thing on the opposite wall.
Measure the distance between the two chalk lines at several points. If the distance isn't the same throughout, adjust one or both lines so it is. Split the difference between both ends of both lines when moving them, so there's as little movement as possible off your original 1/2 inch marks. (For example, if one side of the room is 1 inch wider than the other side, move the both lines at the wide end 1/4th inch inward, and both lines at the narrow end 1/4th inch outward, rather than making all the adjustments at one end.)
Lay your floorboards once the distance between the two lines is the same at all points. Use the lines as your starting and ending points for the long edges of the boards, leaving a gap of about 1/2 inch at the walls on either side. The gap will be covered up when you re-install your floor trim at the end.