I'll set this up, load the pics later.

1. Measure your old window carefully. Measure at the biggest distance, and account for extra molding, framing, etc that some older windows might have. You want an accurate measurement of the rough opening, as most windows will be slightly smaller than this when you order them. If your window correponds to something you can get off the shelf, all the better.

When I have a bunch of windows to do, I would rather order them. I can get the custom size I need, quality is better, and I always ask for double latches. It's a lot better to keep the window square and energy efficient that way in the long run.