Your best bet, start by drawing it out on graph paper. I'm not sure I could have managed a couple that I have done without graphing it first.
Do you mean you want to make the "81" out of the t-shirts? You could do half square or snowball the blocks to make your "8", look up patterns for numbers to get some ideas. And I'm not sure what you mean about sewing them "sew them on the numbers so the sewing doesn't show." The idea of a sashing is a good idea, you could use white or a light grey 1/2" to an 1" to make it more defined.
If you've not made a t-shirt quilt before you should do a search on QB for other suggestions and you can look up YouTube video's as well.