Minniesewta ... when you buy your fabric it is folded on the lengthwise grain .. if that is a good fold, and you can tell by checking the selvedge and making sure there isn't any wrinkle or wobble in the fold line you can get it ready to cut .. but if there is anything crooked, straighten it out before making your square up cut .. I will iron that center fold clear out if it's kinked. Now that you have your center, you can fold it edge to edge, lay your fabric to the right and measure from the left. I lay the fold on a horizontal line on my mat, check a couple of times, and then square off that left edge using a vertical mat line that is pretty close to that edge. Once that is straight, you can line your LONG ruler up at the width you want, say 2 1/4 for binding strips, and holding the ruler FIRMLY with your left hand, cut along that right edge holding your rotary cutter nice and straight. Always use sharp blades in your cutter - and close it after each cut. Now pull away the strip you just cut, measure again, and proceed just like the first one, don't slide your fabric until after your 3rd cut .. then, make sure your fold is still on a horizontal line, check to see if your cut edge is still straight on the vertical line - if not, trim it before cutting another strip. BTW- this cut is called WOF - width of fabric, meaning selvedge to selvedge. Keep your ruler nice and tight to the fabric .. if it feels llike it is sliding - you are not holding it straight down, but letting your hand go to an angle, and that will push the ruler out of alignment.
I probably cut 20-30 strips a week - and I learned the hard way!