I have a 6 1/2 by 24 ruler. How do I cut 8 inch squares! TIA

I would cut a straight edge on your fabric, then measure up 8 inches in several places to keep your cutting line straight, cut a strip, measure it into 8 inch segments marking it in tow places to keep your lines straight and then cut.

Well you make you a templet out of paper or card boardthat is 8inches
square an set it down an then place ruler down where your pattern ends
an cut a strip that is 8 inches wide.Then you use your ruler to measure
eight inches from edge an cut it.An then keep on an go clear across the material.If materialis 36inches wide an yuor material is folded you'll get two cuts each time with four around 4 inches left.Dont use the edge of materialcut it off about 1/4 inch off before you start cutting.OK

i've used my 6x24 ruler but butted another ruler up next to it measuring 2 inches more. or 2 1/2 if need be. just let the other ruler push the longer one over till you have the measurement you need and cut. slide the other ruler up as you go.

You might try folding your fabric in half. Measure 4" (on the 6" side) by 8" then unfold and you have an 8" by 8" square.

If you have a cutting map with marked lines, you can align the cut edge of the fabric and your ruler with those lines. I have several rulers so, in a situation like this, I just butt two rulers together to get enough width.