So, step 1 is to find an old piece of clothing that you no longer like.
Step 2, cut off the arms and turn the shirt inside out. Start pinning the left-over piece of sleeve on the shoulder part.
Step 3, if you want your shirt to be shortened, fold the bottom half up and pin it wherever you want it to be. (Note, this steps takes ages for impatient people and nearly caused me to quit on the whole project, it's difficult to make the vest the same length along the entire bottom.)
Step 4, admire your work so far. Looks pretty good?
Step 5, try it on and try to look nonchalant about it, despite the pin needles sticking into your shoulders. (That's why it's only half-on, there's only so much pain you can take.) Also, hello there dark roots. Happy to inform you that they're gone after today's hairdresser visit.