If you want to make lots of half sqs. use Eleanors Burns method.
for 2 1/2 in. use 2- 6inch squares right sides togather. Mark an x with a pencil and sew 1/4 inch on each side. Cut 3in in from each side then thru the middle of the x lines. Square up. You will have 8 half sqs. For 3 1/2 use an 8 inch sq. You can easily judge size this way.
If you only need 4 use a strip 6 x 3 draw a line at 3in then draw a line from corner to corner on each side of line so it looks like a v with a line thru the middle. Sew on both sides of v line. Cut at the 3in line then between the v line. This is hard to explain without pictures but it is very simple. This method only needs a small amount cut off for squaring up.
If you have high speed internet I recommend watchin video on Qnntv. It show Burns all day every hr on Thurs. And Fons and Porter all day Weds.