Watch your speed. It should be high anyway, with a heavy plane.
Flaps let you come down steeper more than slower.
Getting slow with flaps is just as bad as getting slow without them.
As for amount of travel and how much to trim, start with about 30% down, and at a decent altitude, but not fast, drop them. Observe which way the plane pitches, and how much elevator you have to hold.
Then pull the flaps up and land, and adjust the elevator mix to the same amount of stick.
For the next flight try 50-755 down flap and observe the elevator.
I generally just hold the elevator required, I don't mix it in.
Back at altitude, check what happens with full power. Sometimes the pitch-up is too much to control with elevator, only pulling the power back will prevent a stall-spin.
If that happens with your airplane, don't do that no more!
Flaps are not a cure-all for making a heavy plane fly slower, or better. It still must be flown properly.