I have no idea how to knit a jumper/sweater, but I bought a knitted jumper at the thrift store, and then used duplicate stitches to create the letter I wanted. Got the idea from this video, and figured that even I could do that. Took me some time to figure out what kind of stitches I could use for that particular jumper, but just try and fail, and then try again.
I chose the letter “S” from my last name, since the letter “I” can be kind of boring. But maybe in the future I will create one with the “proper” letter.