Focus on four things: board speed, timing, carve, release.
If you have enough board speed and carry it straight downwind then you kill your apparent wind and get plenty of time to do whatever you want with the sail. With enough speed you can duck it twice.
It is better to duck early than late. Ducking too early can put the clew in your face. Ducking too late usually means losing the sail as the apparent wind picks up on you. Aim for a quick duck just before dead downwind.
Maintain a consistent carve as you duck. I do this by leaving my feet in the straps and switching them after the sail is taken care of. If you stop carving then you you will end up with the sail luffing from the clew and find it very difficult to grab far enough forward on the new side of the boom.
Release means to leave or even throw the clew back over your old forward shoulder. This will leave the new boom right where you want it. The main symptom of bad release is the mast hitting the water.
Minimizing hand movements will make the whole thing easier. When you cross the front hand over the back, reach well back on the boom. Release the old back hand, pull the old front hand over your head and to the full extension of your arm behind you. Reach as far forward as you can with the new front hand and put it on the boom. There are only three boom grabs. Two are on the new side.
Alternate version: duck really early, the moment you turn onto a broad reach with plenty of speed. You are then clew first and at your leisure to get the board on the new tack without worrying about the sail. It is better not to try to alternate version while overpowered.