Sandpaper! Get a piece of 220 or finer (larger number). I cut a piece of 1/4" plywood and stuck it down with double stick tape, but I gave a sheet to my girlfriend who just used a clipboard. It really keeps the fabric from distorting at all and because you can use a bit more pressure with your marking tool, the lines are darker & more visible. For 90% of my marking I can use a #2 lead pencil.
I also purchased the Omnigrid marking rulers with a coupon. Don't have to look for that 1/4" line. Don't really need these, but it sure makes it easier.
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