@Sara-S Pretty much covered by all the other posts, just my penn'orth now!
Check the grain - get it as straight as possible, and check regularly that it hasn't drifted. With a sharp blade you can cut up to 4 layers (carefully!)
Find the trick that works for you to align the edges when sewing - seam guide, bit of tape, whatever. Cut 3x2½ squares and sew them together using your best effort at a scant ¼" seam. the centre square shoule be 2" between the seams, more and you aren't taking in enough, less and you're taking too much.
Cut your pieces a thread or two too large and trim the sections to the right size - not 100% reliable and tedious to trim lots of edges, but sometimes just that sliver off the edge will make it fit.
Now, I like the Omnigrid rulers with the yellow markings either side of the actual size line. Cutting with the outside yellow line gives you that thread or so to play with. Using the mat grid requires careful postioning and, again which side of the line do you cut?
Keep the cutter blade sharp - you can get sharpeners which will work for a few times, but sooner or later you need to change the blade, at which point you wonder why you didn't do it last week/month/year.
If you are cutting wof strips, make sure the selvedges are properly lined up or you will get a V-shaped strip. When cutting long lines, take your time and pause and walk your hand up the ruler every few inches to keep it steady. Some people use a handle to hold the ruler - one of those shower handles you attach with suckers.
If you're making a pattern with alternating plain squares, don't cut them until you've made the pieced blocks. Check the sizes and if they've come out bigger or smaller than expected, you can adjust the size of the plain squares.
Remember it's the 'p'-word. Practice. Perfection is only for God.