The roof, that is. Not that I think shingle manufacturers warranties are worth a whole lot, but manufacturers warranty's are nulled ifthe shingles are not installed over bare sheathing. Plus, you'll get the best looking, and performing, installation over bare sheathing vs adding a second or third layer of shingles on your roof.
It's a pity to do a fine shingle job only to have to rip a few shingles off down the road to fix flashing.
So strip, then flash.
Not a video, but a good book:
Visit the shingle manufacturer's website. Actually READ the istructions that are printed on the inside of the shingle wrapper. Adhere to the nailing pattern.
Technique-wise, once you finally start pounding nails, open two or three bundles and work from all three as you go. Some bundles may have a subtle color difference from others, this mixing will ensure that you don't get that sore-thumb-looking stripe of color running up your roof.
If you can move up to a laminated-type architectural shingle, they are so much easier to lay in terms of not having to worry about the tabs lining up as you work up the roof with 3-tabs.