Multicolored tailor's chalk at Hancock fabrics, ~$8.95. On the wall near the velcro, etc.
This REALLY is a GREAT help for marking the cutting lines and marking the sewing lines for your project, especially for the baffles on a quilt.
The package comes with a holder like a mechanical pencil, and many sticks of white and several sticks of other colors. The chalk sticks to the ripstop or silnylon long enough for you to do your work, and then it rubs off. It will also mark no-see-um, but you have to have very good eyes to see it. It also comes with a pencil sharpener if you want a really fine line, but rotating the chalk as you draw is good enough.
For a quilt, mark the lines for each baffle, and you can then see the line through the no-see-um as you sew. I was able to do all of mine without having to pin anything. (Someone else suggested doing all the measuring on a long piece of - straight - wooden molding, and then transferring that to the fabric. GREAT idea!!!)
Carpenter's square, at homedepot ~ $6.95, is the way to go to mark 90* angles. I covered a piece of 4x8 plywood with tyvek and set it up on sawhorses. Tape 1 corner of the fabric to the tyvek. Carefully smooth out the fabric and tape along 2 sides. After marking the square with tailors chalk, use a steel meter ruler (don't remember where I found this) to continue the lines as far as needed. For a big project you will have to move the fabric across the plywood as necessary, but always retape!