The moment you get strapped in, you break one of the biggest barriers of windsurfing skill. Getting into the foot straps is straight forward, you need to have a good attitude and commit yourself. When you are strapped in and using the harness, you are at the level of proper planing for the first time, you'll experience one of the biggest thrills and achievements in your life. Although it's straight forward, there are a few pointers we need to keep in mind, so let's take a look at them.
First it depends on the gear, if you are on a big board with adjustable straps then you have a major advantage. This is the best way to learn because you can simply adjust your straps so that they are closer to the mast. This way you can slowly move the straps back as you practice and get used to the feeling of it. You also don't need to be planing to start out like this and you can practice in relatively light wind.
If you are on a smaller board without adjustable straps then you'll need enough wind to get planing, if you try to use the straps in light wind you'll just sink the tail. Okay, so we have the wind, now what?
Body position. Body position is the first key, as you sheet in and increase your speed you'll need to be simultaneously moving back towards the straps. Moving back helps to create lift and when you create lift less of the board is touching the water, when that happens there is less resistance (drag) and more speed.
Now that you are powering up and moving back, your front foot needs to slip into the front strap first. Once your front foot is in, your back foot will naturally want to go into the back strap, don't be scared just do it. The first few tries will feel a little strange with the board feeling slightly out of control, that will go away with practice as you learn to move in faster.
Keep your knees bent and remain relaxed, if you are stiff it makes everything bouncy and it's a pain getting into the straps on a bouncy board.
With you back leg bent more than your front, lean back and get comfortable. You'll feel a sudden release and you'll be planing in the straps.