You're probably getting off-grain. Fabric folds a little easier and stays a little straighter if you stay on-grain.
If you have enough fabric to sacrifice a few inches, the easiest way for me to get on-grain is to RIP the fabric! I like to do a nice rip across the width of the fabric (rip off one of the cut ends) and that guarantees I'm on-grain. But this is optional.
With or without ripping...when you fold the fabric, hold it up by the folded edge and let the fabric hang. Look at how it hangs. If it doesn't hang straight, you're off-grain. Adjust your fold until it hangs straight. If you ripped off an end, your torn edge SHOULD line up; the other cut end may or may not.
When it comes time to cut the fabric, I trim off the torn edge about two inches down from the rip (because ripping pulls and damages fibers) and toss that part. So you'll lose 2-3 inches of fabric with the ripping method. (More if your fabric was initially cut very crookedly.)
When you cut your strips, line your fold up with a line on your ruler. Trim it square, and then cut your strip. Don't just assume you're staying straight by measuring from your freshly cut edge, either; every other strip or so, re-align that ruler line with the fold and trim it back square if necessary. Most of us end up a little "off" after a couple strips, so re-squaring will help with the crooked cuts.
It's not easy!! It does take some practice but you'll get there.
