Here is a quick little method for easily trimming your half-square triangles to size.
First, make sure you have sewn the block a little bigger than you want it to be after trimming.
This one is about 5" on a side and I'm going to trim it to a 4 1/2" square.
Do not iron it open! Keep it just like this.
Both fabrics on the right side edge of the triangle must be lined up evenly with each other.
The right side will not be trimmed.
The fabric along the top edge is does not need to line up evenly, because it will be trimmed.
Next, line up your ruler as shown here:
Cut off the fabric along the top edge of the ruler.
That's it! Just one cut.
Iron the block open and you're done.
