I will like to share the advice of my Roofing teacher, On top- on top- on top.
I am a licensed roofer and having done many repairs i know at first you can get overwhelmed once you start removing things. slope helps too : ). The roof is easy like putting in flooring except you want all seams or overlaps to be on top of the lower anything where water may at some point flow. I use the Pry Bar Like he has in video and you can run it vertically along seal across shingle sometimes easier than straight up between. hammer helpful to break seam. You can get a hammer with a magnet and not smash your fingers. or hold nail with both your finger pads up back of hand down. hurts less and rarely breaks a nail. or turns it black. take nails out and save on roof beside you, usually can reuse. Care must be taken with lifting old or cold shingles to nail or get at nails they will be brittle and break easily sometimes.
If you tear a shingle you are not replacing you can seal with cement easily. seal all nails go easy on cement or you will have a lumpy mess. Take pictures as you go to show what you did. I personally always have the roof cement available. you want to get it and devote a caulk gun to it maybe, And you want it warm. if it is cold out put caulk in truck and heat it up or you will have a terrible time getting it out. I bought two or three caulk guns the first 30 degree day I was trying to seal a transition seam for the crew in a hurry to beat the inspector, I thought the gun was broke I couldnt get it to work. Went to store and bought new gun, piece of junk wouldn’t work! went again… got it done but took longer than if I was thinking more than I was in a hurry…ha ha ha. two cents hope it helps. Have a great day!