Back in middle school (grade 7/8), all the kids in his class were forced to put on a mandatory Christmas Play for their parents (or, in Peter's case, aunts and uncles!). Since Peter refused to act, and he was actually pretty bad at it, he was volunteered to do backstage stuff.

Assigned to help make the costumes, this was Peter's first opportunity to learn sewing and costume-making ... which is how he knows what he's doing when he sews his Spidey costume. He is actually a deft hand with the needle and thread!