Now don’t start thinking you can’t do yourself. Just watch and you’ll be doing yourself in minutes. Just remember to only touch it once and not over and over. Every time you touch it you’re pulling off way more than you put on.
Before you do anything make sure all nails are not raised up, make sure they are well seated in the rock. Spread a nice amount of joint compound in the corners but not too much. You just don’t want to see the sheetrock under it.
Fold the tape in half, then gently insert the tape in the corner. Now with a 6-inch knife push / pull the mud (joint compound) back out while the tape lays nice and flat.
Do this to both sides (left and right) making sure you don’t leave any big junks behind. Wait till it’s good and dry and repeat. That’s it, now you’re a pro.