These are SOO easy. The picture I pinned doesn't have a tutorial so I'm making one.
a t-shirt, two contrasting t-shirt scraps (mine are left over from the lowercase c capes) pins parchment paper, a sewing machine and thread. (the sewing machine is optional I guess if you have a needle and thread and a lot more patience than me)
Trace/draw your number (or letter or other symbol you want on the shirt) onto a piece of parchment paper (or any thin paper I guess even notebook paper would work, but I used what I had). Layer the paper, fabric scrap #1 (main color-red), and scrap #2 (outline-blue) on your shirt and pin.
Sew around the number (or letter or other symbol) right on the outline through the paper. I used a contrasting thread because I already had a bright green bobbin.
Trim the first layer of fabric around the sewn outline
Trim around the second layer slightly bigger to create the outline. And you're done!!
If you want to be really impressive, remember to take a picture of the birthday boy wearing his shirt.....