Half square triangles are a patchwork staple.
There are a few ways to make half square triangles (HSTs). I am going to show you a technique that is good for beginners, that doesn't involve having any stretchy, bias edges on the sides of your square.
To make a 3 1/2" HST you will need two contrasting squares measuring 3 7/8"
On the wrong side of one of the squares (usually the lightest colour) draw a line diagonally from one corner to the opposite corner.
Place the 1/4" line of your ruler on the line you have just drawn and draw a second line.
Repeat to draw a third line on the other side of the first line you drew.