OK 16 stone...this makes quite a difference and explains some of the issues you are having!
OK we'll go through the whole sequence:
1/ Place your feet with your arches directly over the fore and aft centerline of the board.
The front foot goes slightly (due to your weight) in front of the mast foot (place the mast foot in the
middle or behind the middle please to keep your mass over the widest and most buoyant area of the board).
The rear foot goes somewhere behind the mast foot... about a shoulders width behind the front foot.
At 16 stone, you will need to manage the fore and aft balance of the board as much or more than the side to side (rail to rail) balance.
Your back needs to be directly toward the wind with your board lying directly accross the wind with the mast down wind and perpendicular to the boards fore/aft centerline.
2/ When you have your feet placed correctly and you can stand up straight, with one hand on the
uphaul line (rig lying in the water downwind of the board).
Pull the rig so the mast is perpendicular "T" to the board.
Squat down and move your hand further out the uphaul.
3/ Slowly stand up from the squatting positon and bring the mast out of the water.
Continure to bring the mast up out of the water until you can grasp the mast (below the boom) with your
new front hand (the hand that will be nearest the front of the board).
4/ Hold the mast directly in front of your body with the end of the boom straight out (T position alignment again). If the board begins to turn nose into the wind, lean the sail towards the rear to stop the turning and return the board to directly across the wind (T positon).
If the board begins to turn nose downwind, lean the mast toeards the front of the board to return it to the
perpendicular "T" position.
5/ Move your rear foot a bit further behind the mast so that your rear foot is about a full shoulder width behind the mast.
Bring your front foot from ahead of the mast to just behind the mast.
BE SURE TO LEAVE THE SAIL/BOOM PERPENDICULAR TO THE BOARDS CENTERLINE.
DO NOT MOVE THE MAST BACK AS YOU STEP BACK!!!
6/ Position your front foot about 4-6" off center of the fore/aft centerline and slightly behind the mast foot
with your toes pointing forward toward the front of the board.
Place your rear foot so you heel is on the boards fore/aft centerline with the toes pointing about 45 deg. foreward of straight across the board.
Feet need to be about the width of your shoulders apart.;
This will put your center of mass directly over the boards fore and aft center of buoyancy.
RESIST ALL TEMPTATION TO GRAB THE BOOM WITH YOUR BACK HAND!!!
Ok, now you almost in sailing position..... check your alignment to the wind and the "T" position.
If you are not standing steady, with both feet behind the mast foot, with the end of the boom pointing straight down wind and perpendicular ("T" position) to the F/A centerline of the board, you need to correct your alignment!
If the board has turned upwind, this means you brought the mast back as you stepped back. Lean the mast/sai forward slightly and the board will correct back to the "T" positon.
If your board has turned downwind, this means you have allowed the mast to fall toward the front of your boars some. Correct this by leaning the righ slightly to the rear.
7/ Now bring your front hand (holding the mast under the head of the boom) across in front of your front shoulder.
Try to bring it across to the point that the rig balances on the mast foot. You can check this by letting go of the mast momentarily. If the rig tends to fall back toward downwind (leeward side) you haven't brought it far enough across to the balance point.
If the rig wants to fall towards the upwind side, you have brought it too far upwund....correct for this until your rig "balances" on the mast foot.
Again...RESIST ALL TEMPTATION TO TOUCH THE BOOM WITH EITHER HAND.
GRABBING/TOUCHING THE BOOM WILL SCREW UP YOUR ALIGNMENT ("T" POSITION HERE AGAIN)!
8/ Now you are ready to place one or two fingers of your back hand on the boom. about a soulders width
behind the mast and right in front of your rear shoulder.
9/ Move your front hand from the mast up onto the boom very near the juncture of the mast and the boom.
You still have not moved even one inch (perhaps a slight drift downwind, but no movement forward or back.
Your board is still pointing directly across the wind (the easiest point of sail).
10/ Now rotate your upper body (from the hips) a few degrees so the sail begins to develop some pressure. Rotate the back shoulder inboard and upwind slightly and the front shoulderoutboard and downwind slightly.
Your board will start to sail off directly across the wind.
If you wish to sail a little more upwind, move the end of the boom down slightly toward the water to incline your rig slightly to the rear.
If you wish to sail more downwind, extend your front arm slightly to move the end of the boom up and the right slightly forward.
You can see the various positons and alignments on this US Sailing instructional website:
You need to look at:
P.S. At no time do you step forward or radically rake the rig forward.
It takes a little time for the board to turn to the correct alignment (T Position again) but the
alignment is critical to learning to windsurf at the basic level easily without alot of drama.