1. Start with a well-fitting t-shirt (or start with one that is too big and re-size it, but that's been covered in many other tutorials), a couple of yards of t-shirt/knit fabric, and a separating zipper long enough for the front of your shirt. Alternatively, you could use a long-sleeved t-shirt for the extra fabric. You'll also probably want a hoodie on hand to use to make your patterns for the sleeves and hood.
2. Using a seam ripper, carefully remove the sleeves from your t-shirt.
3. Lay your already-existing hoodie sleeves flat on a piece of newspaper or pattern paper and trace around the sleeves to make your pattern. Make sure that the sleeve opening size of your pattern is the same as the sleeve opening size of the t-shirt from which you just removed the sleeves. Don't forget to add a little bit of room for the seam allowances!
*Note: I got all fancy and added ribbed cuffs to the bottom of my sleeves, so I made the sleeves shorter than I otherwise would've and didn't hem the ends. If you're not adding ribbed cuffs be sure to make your sleeves long enough and hem the ends before you sew them into a tube. If you are, you can add them at any point, and if you're like me and this is your first time doing them, this link is very helpful*
4. Lay your sleeve pattern flat on a folded piece of fabric and cut out your sleeves. Mine ended up looking like this:
5. Fold each sleeve in half lengthwise and sew up the arm seam, creating a tube. Then pin your sleeve tubes to the t-shirt's sleeve openings and sew.
You could stop here and have a long-sleeved tee, but continue on to add a zipper and a hood.
6. If you're anal like me, take a ruler and measuring tape and chalk and mark halfway down the middle of your shirt, then cut up that line. Or just eye it, if you're good.
7. Put in your zipper. I am not going to give instructions on this because I proved myself completely incompetent at installing a separating zipper, even though I've put in many zippers before.
*Optional step: Before putting in the zipper, realize that the blue one (in the first picture) that you so-conveniently already had is not, in fact, a separating zipper, but a regular zipper. Realize that all of the fabric stores will be closed at this hour, so get annoyed and give up for the night and go buy a separating zipper the next day.*
My boyfriend thinks I should've stopped here and not added the hood, but I think it's much cuter with the hood, and if you agree, continue on....
8. My hood is lined so that you can't see the wrong side of the fabric at all, because I think it looks nicer this way. To make a lined/reversible hood, take your already-existing hoodie and lay it flat and trace around it to create a pattern, or just enlarge and use mine:
9. You're going to need to cut out 4 pieces of fabric from the pattern you just created above. On your folded fabric, cut this piece out twice.
10. Sew along the top and back edge of each pair of hood pieces, creating two hoods.
11. This may be a bit confusing, but I'm trying to make it as clear as possible, so bear with me. Right sides together, place one hood inside the other and sew along the front edge, like so:
It should look like this after you turn it rightside out:
12. *Note: If you want to have a drawstring in your hood, add buttonholes or grommets on the sides of the top hood fabric layer before you continue with this next step, and when you stitch the next seam, start about an inch from the front to leave room for channel for the drawstring. Add an additional seam around the front of the hood to create the drawstring channel, and thread your string through the holes.*
Now take the hood, and from the outside, stitch in the top seam from front to back.
13. Finally, take your hood and sew it to the collar of your shirt. You may want to remove the shirt's already-existing ribbed collar, if it has one, before you attach the hood, but I didn't.
Yes, I accidentally sewed over the tag so it was all askew, but I took out the stitches and fixed it before proceeding.
*To finish, I ironed the preceeding seam towards the bottom of the shirt, and then from the right side, I stitched in the ditch of the ribbed collar-to-shirt seam to hold down the seam I'd just made.*
14. Admire your creation, and show it off for the admiration of your friends, family, acquaintances, cats, random strangers, etc.