Accuracy is important in quilting. Trimming our HSTs to size helps us make sure that are points are sharp and our rows are straight.
In most quilts, you will either make your HSTs slightly bigger than their intended size and trim them down, or you'll make them at the correct size and trim to remove "dog ears" and any distortion along the edges.
Here, we'll start with an oversize HST block and trim it down to size.
Line up a square ruler to your HST block. Match the seam of the HST to the top right corner of the ruler.
Think about the unfinished dimensions of this block -- the dimensions you want your HST to be after trimming. Make sure that the untrimmed sides of the HST under your ruler extend beyond those dimensions.
Here we are trimming this block to 3" square, so we want to make sure the HST reaches past the 3" mark at the left and bottom of the ruler.
Trim the top and right sides of your block.
Turn your block 180 degrees, and line up your ruler again, this time to the desired size of your HST. Match the seam of your HST to the 45 degree mark on your ruler, or make sure the seam is running corner-to-corner.
Here, we're trimming a HST with an unfinished size of 3", and we want to trim our block to 3" square.
Trim the top and right sides of the block.
And you're finished! A neatly trimmed HST, ready to be sewn into your quilt.