Yep. Had this problem too. I fold fabric salv to salv. Press the seam. Fold the seam just up to the salv so the salv peaks above the fold about an inch. Using the marks on my 2" ruler, I place the ruler so the marks are along the bottom fold. Don't move the ruler. Look at the marks at the too of the ruler. If the folds are on the marks, don't tilt, the fold is straight and I can cut. You will have to trim and toss out a little bit.
If you are cutting from a large piece of fabric, you may have to keep adjusting your folds to keep everything straight. Sometimes fabric likes to creep. 8)
Please don't get frustrated. Quilting is a wonderful hobby. I've found playing with fabric and making something pretty out of fabric is great therapy. I'm a caregiver for my bedridden mother. Playing with fabric helps me get through the bad days.
Julie in NM