I am gonna guess you're talking about the tutorial from missouri star quilt co.
http://www.youtube.com/watch?v=3gKvdrexKE4
I would find this ruler
http://www.missouriquiltco.com/notio...-hultgren.html I got mine at hobby lobby with a 40% off and it was less than seven bucks I think, you can also get one at joann's with a 50% off coupon for even less! it cuts MANY different sizes The only thing is it is EASIER if you use a 28mm rotary cutter. However I have always used my 45mm cutter you just have to be more careful! I am to cheap to buy a different cutter just for that ruler!
To find where to mark to sew you can do it this way... If you are going to make several of them I would get a piece of template plastic and draw your circle the same size as your circles AFTER they are sewn and then poke little holes where you need to mark! or just cut out a square of plastic the size you need it, then you can just lay it on top of your sewn circle and trace it!
fold your circle in half twice
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press it so the lines are visible turn the circle so the lines form an X in front of you
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line your 45* line up with one of the lines with the very point just on the inside of your circle! you want the sides of the ruler to match up with the other line at the same point draw those two lines
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turn the block around and do the same on the other side then you will have a square inside your circle the four corners of your square are in the spot that she marks on the template.
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