The easiest example I can think of to demonstrate squaring a block is Half Square Triangles that have been speed pieced--where you lay 2 7" squares right sides together and draw a diagonal line on the back side of the lighter one then sew a scant 1/4" away from the line on both sides. Cut on the line and you now have 2 slightly wonky Half Square Triangles so you want to trim them to be the same size--6.5 inches unfinished.
After pressing--not ironing--the blocks and letting them cool a few minutes you lay one on your cutting board and line up your 45 degree diagonal with the line on your ruler while also trying to line up with the 6.5" marks but on this first cut, you're really just making sure you have enough going over the 6.5" marks because you're going to turn it and trim again--squaring up all 4 sides in 2 cuts.
So--you have your block laying on the cutting board with the 45 degree line matched up with the diagonal seam and the left and bottom edges of the block are just over the 6.5" marks on your ruler and the top and right sides are probably about an 1/8th of an inch peeking out from the top and right side of the ruler. You'll trim off the part peeking out. Then move your ruler and turn your block 180 degrees. Lay your ruler back on it matching up the 45 degree line with the seam again and this time your left & bottom edge of the block should exactly be on the 6.5" marks on your ruler before trimming the part that is peeking out from the top & right side of the ruler.
If you are left handed--reverse the right & left directions above.
I thought there was a tutorial on this site for Half Square Triangles so you might look at that also. Squaring up blocks involves finding the exact center of more intricate blocks and as a previous poster mentioned--blocks with points should be pinned, spritzed and blocked (like you do with crochet & knit sweaters) or you will end up trimming off points.