I used a t-shirt that was too small for me because I wasn't sure how it would look so if you use one that fits it will look better (:
1
Get a plain t-shirt and lay it out (Ignore my knees!)
2
Draw 4 lines with the chalk
3
Draw the spiderweb from there!
Cut along the curved lines and a hole right in the center.
5
It should look like this when you're done! :)
6
And look something like this on :D