I'll be making some alphabet quilts where I use fabric for each letter and then quilt in the appropriate letter as well. Lucky for you, X isn't too terrible to figure out or coloring books are a great place to find things like that. In a t-shirt quilt I'd probably outline an X through each individual t-shirt block, with the width at least 1" wide.
Because of my vision issues I often mark my quilting designs on paper and quilt through it. When/if you use paper to sew/quilt through, I really like the parchment paper I get at the dollar store. A roll is 1 foot wide by 25 feet long, it's clear enough to see through and tough enough to hold up. When you need a bunch of template sheets, the easy answer is to draw out your design and then tape together (using blue tape with a tab or two on each side) maybe up to 6-8 sheets of paper and simply sew through them without using thread in your machine (I change my needles often and always after sewing through paper). You will be able to see the punches just fine. When you are just marking the designs on paper, I'd use a reasonably large stitch (8-10 per inch), when I've used small stitches it pretty much cuts through the paper and I have problems with my presser foot going through and tearing the paper before I'm ready. It does take time to rip off the paper and it's a bit messy but it lets me do complex designs that I can actually see -- and the nice thing is that there are no marks on your quilt top to take off or for people to see my lack of accuracy!