Well .... Never sure what is "obvious" ...
1. If your framing is uncovered, you should see the header, jack and king studs have to be re-framed to accommodate the new door. You know how, right?
2. You will have to cut through the exterior siding at the correct dimension for the rough-in of the new door. Easiest way would be to keep on of the door sides and just push your framing over for the new door width. Got that measurement? Got the saw? Only cut the siding and sheathing, not the framing. Right?
3. Depending on the load, you might want to support the garage framing with a temporary brace to keep it from sagging. Check?
4. Having cut the new hole, now cut the existing framing out as needed, re-frame, install the new, beefy header, and double check the rough-in dimension so it fits the new door.
5. Don't forget your flashing and building wrap.
6. Now you can install the new door. You know how, right?
Have a great time!
OH, here is a basic framing video ... He starts talking about the door at 10:30 but it is fun to watch the whole thing ...