IMHO, you have to get into the harness when the wind is very light, like <10 knots. Your muscle memory will grow. Use a big board (if yours is 75cm wide than its ok) and a small sail with less battens (maximum 5 i guess) like 5m2 or may be 4,5. try to fall in both sides, never let the boom go, always hang to it even when catapult (it will save you and your kit). start hooking in a 45 degree to the board, and while speed up, u have to close the sail, that means you have adjust your stance. That means you have to become parallel to the board when in speed but at an angle when moving slowly (adjust your position according to the sail angle to the board, try to stay parallel to the sail).
The length and the position of the lines are important. Do not use a short harness line, because you have to spread your arms resulting a closer stance to the sail, and not an upright position for the sail, may be resulting in catapults, secondly if you have a short line you have to lower your boom to be able to reach the line, resulting horizontal lines and not be able to commit to the harness and the boom, so a lot of weight go to the board, not able to plane early or not able to plane, spin outs,..
Contrary if you have long enough lines, your sail will be more upright, your stance will be far from the sail, you can commit to the lines, you can use higher boom, all resulting fast sailing, early planning, less spinouts
And why u get into the straps if you're not on the harness anyway, either harness first then go to the straps, or first go to the front straps then harness, then back strap, but to be able to use smaller boards i think you get to use to get into the straps when the board have enough speed (just before or after the planning)