You can not windsurf well without a harness. Period. If you want to know why, see the For Nerds box at the end of this section. You need a harness. You need to know how to set it up, and how to use it.
There are two kinds of harnesses: Seat harness and waist harness. For your first harness, I recommend a seat harness. The reason is that a seat harness promotes a better high wind stance, at least while learning. With a seat harness, you are less likely to bend at the waist. You do not want to bend at the waist. Later, after you have a good high wind stance, you might want to try a waist harness, particularly if you get into freestyle or wave sailing. When you purchase a harness, you will also have to purchase a spreader bar with hook. And of course harness lines (discussed below). Some seat harness are integrated into board shorts, some waist harnesses are integrated into PDF (life jacket). Those are fine if they fit well.
When you put your harness on, all the straps must be tight. This is particularly important for waist harnesses. Very tight. You do not want your harness to move up your body while sailing. If it creeps up your body, or the straps that hold the spreader bar/ hook become loose, the hook will be in the wrong place.
There are several things you need to adjust before you hit the water: Boom height, harness line length, harness line placement on the boom. Boom height and harness line length, together, determine determine how low the harness lines hang. The way to think about it is that the bottom of the harness line loop of your harness lines must be low enough so that it's near the hook to make it easily hook in. However, if the lines are too long, you will not be able to get downward pressure on the boom, and the harness lines will not be effective (might as well have saved your money and not bought the harness). Where the loop of the lines are located is determined by the harness line length and your boom height. In the pictures below, the harness lines are the same length, but the boom is in a different position. For me, on the left, the harness line loop is too high and on the right it's too low.
Boom Height? Styles change, like women's skirts, boom heights go up and down over the decades. You have to establish where you like to have your booms. Here is one guideline: shoulder height (or a bit higher) when you are comfortably sailing (not standing on the beach). This is the one position where you can comfortable lean back against the force of the sail without using your triceps or shoulder muscles. Experiment a bit. When you find a height that you like, come back to the beach and measure it! Measure it from the base of the mast with a tape measure or use your body as a measuring device. I find that if I hook my arm over the boom, and stretch until my finger tips are 1/4" from the bottom of the mast, I am a happy camper:
Harness line length. Fixed length harness lines come in lengths from 16 to 26 inches. Here is where adjustable harness lines might help you, since you do not know exactly how long you want your harness lines, yet. If you do not want to spring for adjustable lines, try a 1/4 inch rope, and carefully tie it on to your boom. Again, you want to be able to hook in, so the loop in the line cannot be too high. However, you need to put your weight on the lines, so the loop of the lines can not be too low.
Again, when you find a line length that you like, come back to the beach and measure the length. If you have a tape measure, use that. I have adjustable lines, and I measure with my arm. All of the lengths below are acceptable, depending on your sailing style. The one on the left has my elbow in the loop and my boom just above my watch. This is a moderately short setting (for a for someone not planing). On the right, my whole fist fits inside the loop. This is pretty long. When you first start to use a harness, you will might want to have your lines on the long end of the spectrum.
You have to be precise. Variation of more than 1/2 inch will throw you off. The goal of all this fuss is so that you know precisely and intuitively where your harness lines are, so you can hook in easily without looking. After you hook in, you want your harness line to take almost all of the pressure off your arms, and provide lots of Mast Base Pressure (MBP, see section at the bottom of this page).
Placement of boom. There are two aspect to this. First, the two harness lines should straddle the sail's "balance point." There is one place on the boom where the force of the wind from the front of the sail exactly matches the force from the back of the sail. Here is how to approximately find that location. Stand you sail up in a windy spot. (Be sure no one is downwind of you). There is one spot where you can hold your boom with one hand. That is the balance spot. Unfortunately, that might be slightly different on the water, where the wind is stronger so after you start to sail, you may have to make further adjustments to have perfect sailing (harness lines on the balance point). Here is the rule: If you front arm is tired, it needs help, so move the lines forward a bit (toward the mast). If you back arm is tired, move the lines back a notch to assist the back arm. If you are perfectly balances, windsurfing takes no upper body strength.
How far apart should your harness lines be? Here is the general rule. The closer the are together, the more accurately you will be able to feel if you are on the balance point. However, if they are very close together and they are not on the balance point, it will be hard to sail and you will need to adjust them more frequently. Here is a compromise. Start with the two harness lines no further apart than the width of two fists holding the boom. As you get more use to the harness lines, move them closer and closer together. In most conditions, I have my harness lines touching, but that might be a bit extreme for the neophyte harness user.
Using your harness lines. Hooking in is not hard if you have the lines the right length and your boom the right height. Head off the to a beam reach before hooking in. After you hook in, either sit-down or lean back. You want to make the harness take all the force from the sail, not your arms. If your harness lines are around the balance point, you should be able to let go with one arm or the other. Your grip on the boom should be light, and you should be able to "play the piano" on your boom.
The first few times you hook in, you may be "launched." A gust of wind comes along and you are not ready for it. To avoid getting launch, put more weight on your back foot. Also watch the surface of the water ahead of you for gusts and lulls in the wind. That way, you will be prepared for what is coming.
I found this link on harness line use had some very good advice.
If you have the right stance, and you are using your harness correctly, you should be hanging all of your weight on the harness, have a light grip on your booms, be nicely balanced in your harness lines, and have a smile on your face face like the lovely windsurfer shown here:
For Nerds, the secret of Mast Base Pressure (MBP)
How is it that windsurfers are able to sail so fast? On most days, I leave my boating friends in the dust (spray). The harness plays a large part in the speed. Note this happy sailor. Most of her board is out of the water so there is little friction with the water. The last few feet barely kiss the water. Sailors call this "reduced wetted surface."
She is in the footstraps, the footstraps are on the back of the board. Why doesn't the back of the board just sink? If you move to that position, you will just sink the back of the board and fall in the water. There are two reasons she is able to be in the back of the board. (1) Speed: She is getting lift just a water ski boat gets lift. It's called "planing," and it's the goal of a happy, healthy life. (2) Harness lines and MBP (mast base pressure): She is hanging her weight off her harness lines. This is transferring her weight to her boom, mast, and down to the mast base (hence "MBP"). So she is standing in the back of the board, but she has transferred her weight to the front of the board by hanging her weight on her harness lines. You can look like this too if you hand your weight off your harness lines and think MBP.