I've never had any luck making templates from mylar so I use an alternate method. I use the really fine netting like you find in bridal veils. It doesn't cost much and a yard of it will make a lot of patterns. I copy my pattern onto freezer paper, then place a piece of the netting on top of the pattern and re-copy it using a fine permanent magic marker. You can use these over and over again, with whatever marker you want for the fabric your'e using. It even shows up just fine on black fabric. Just put a few well placed pins around the pattern on your quilt top and it makes it nice and stable for transfering.
Having said all that, I hope you're able to find a good method for making your mylar templates. :) I think maybe I just didn't have the patience to keep trying after messing them up so many times! :P