How to make a bodyboard? By knowing how a board performs will allow you to choose a bodyboard for your surfing or for different wave or break conditions.
NOSE WIDTH
This is how wide the flat end of the nose of your board is. Nose width effects water displacement, speed and maneuverability. A mid nose creates smooth flow of water down the rails of the board giving more speed and better edge holding. Larger noses create a larger pivot point for spinning and better maneuverability. However, the larger the nose, the greater the chance you have of catching it on the side of the wave sending you skidding forward on the wave without a board under you, or at best, slowing your progress. Also it increases the reliance on the back of your board for turns. If the nose is not clear of the water when making your turn you are gone! For the beginner the narrower nose lets you get away with more in the way of turns and bumpy waves.
TAIL WIDTH
This is how wide the tail of your board is from the two end corners. A wide tail tends to hold better on waves as well as providing a faster path for the water to follow (hence more speed). Too narrow a tail will promote skating or side slipping of the board (which may not be a bad thing if you are trying to spin your board).Most tails will be half way between the width of the wide point and the corner of the nose.
TAIL SHAPE
Tail shape tends to affect the length of rail on the board, the tendency of the board to catch and the comfort of ride. There are a number of shapes going around, Rounded, Crescent, wing seem to be the most popular. A lot of it is comfort. I’ve yet to meet a person who is perfectly square around the abdominal area (even if you have the six-pack stomach!). You will find that a curved in tail area is more comfortable and will allow you to control the board a lot more with your hips, helping control on the back of your board. Try standing on your skateboard with your feet forward, one behind the other and now try to turn it... it's really hard to control. That's why you stand sideways... to get your feet closer to the edge of the board. Same goes for body boarding; it's no good having just your hands on or near the edge if the back edges of your board have no weight on them. Tail shape is often personal preference. Many body boarders prefer to have extra length in the tail, making Round tails and Wing tails the most popular. If you want to pull endless spins and the like having that bit less grab on the trailing edge of the rail will make the board a lot looser. It will also mean you are a bit higher up out of the water (depending on board length of course!) as your hips are now on the board.
BOARD LENGTH
Tail to nose measurement body boards come in a large range of lengths. A basic rule is - The taller the rider - the larger the board. As a guide the board should come up to your navel, but bear in mind that the longer your board, the less way-out moves you may be able to pull on it. For example, people who are tall yet relatively light will not get the performance out of a longer board that a heavier person of the same height may get.
WIDE POINT
The widest part of your bodyboard is ideally the widest point of the board (wide point) will be just behind your elbows when riding. This will make water disperse behind your face. A wide point above your elbows may help stop the board to nose dive but produce spray hindering your vision while on a wave. The wide point on your board will be relative to the length, tail and nose width. As a general rule the heavier you are the wider the board needs to be... if you don't want to go for a wide board and you are heavy, you will have to go for a thicker board.
ROCKER
This term refers to nose and tail kick. Nose kick generally starts about 1/3 the way down the board and goes up to the nose. The lower the kick, the more speed you will get out of the board, the higher the kick - the less likely the board is to dig in when surfing in choppy conditions. The trade off is between the two. An advantage of nose kick is in coming off the top of steep waves... you can get a better landing and turn from the extra curvature of the board, and get back inside for that perfect tube. Some boards have a flexible area in the nose that allows for variable rocker on the board. When you pull hard turns, the board bends to carve an arc in the water. Tail kick tends to create an air pocket under the back surface of the board slowing the board down. However there is a plus side if you are really big in spins. Tail kick will help prevent you catching the tail during a spin, and will allow you to pull off those tube spins you could never seem to do (though I think I'd be looking for divine intervention for me to pull off a tube spin).