One word will change your life: Thangles! Go to your LQS and ask them how to use them! You'll get a quick lesson and be hooked for life!

This is how I used to do it. I used blue painter's tape in front of the needle. Positioned a ruler so that the needle touched the right side of the ruler. Lowered the presser foot to keep the ruler in place. Checked to make sure that ruler was straight going from front to back. Used a Sharpie marker to draw a line tight against the ruler edge.

If you have a machine with variable needle position, so that you can position your needle a scant 1/4" from the Sharpie line, you are all done. Position your needle 1/4" from the Sharpie line, and you run your squares through with the points lined up with the Sharpie mark.

If your needle is stationery, you need to draw another Sharpie line on the painter's tape 1/4" to the left of the first line. Line up the points of your square with that second mark when feeding them through the machine.

Basically you feed the squares through in one direction, then the other in order to sew two lines, each 1/4" from the middle. Then cut the squares apart between the two sewn lines. Do a couple of test triangles to make sure you have the relationship between the Sharpie line and your needle correct.

This is a lot harder to describe than to do. Used to be able to purchase a card with all the Sharpie lines on them, that you could tape to the bed of your machine. Couldn't find it when I Googled, though; maybe it isn't made anymore.

The nice thing about this method is that you are sewing the bias before cutting; less chance of distortion. Also, all edges of the HST are on the straight-of-grain (assuming you cut your squares on the straight-of-grain!).

There are many ways to make them. Ive never had luck with methods for exact size HST so prefer the oversize method.
I have used thangles, ta da, paper grids but love El Burns method the best.
With her triangle square up ruler you can make alot very fast and they are always perfect. One cut to square up is amazing to me.
For 2 1/2 squares you use 2 six inch squares right sides togather. Draw an X and sew 1/4 in down each side of line.
Cut in half both ways then on lines. You will have 8 HST that just need a sliver trim. Using her ruler with the one cut method it is very fast and accurate. It is different in that you do not open the square but position the ruler on the stitch line and make one cut to square up. And they are not on the bias.