The kite bridle is the arrangement of strings that go between a kite and the flying line. The bridle holds the kite at a certain angle to the flying line. It affects how the kite flies, and whether it flies at all. The point where the bridle attaches to the flying line is the tow point.
I have included an illustration of a bridle on a fighter kite. This is not a diamond kite, but the principle is the same.
This illustrates the basic rule of thumb when bridling your kite: the bridle section that attaches near the nose of the kite must be shorter than the section that attaches to the leg. When you dangle the kite from the flying line, the nose of the kite will be above the tail.
From this basic rule of thumb, you will have to figure out how much shorter the nose segment should be than the tail segment. I usually begin by doing what I'm doing in the picture. I dangle the kite from the flying line and adjust the bridle until the kite hangs somewhere between 15° and 35°, nose up.
Then, I fly it. Either the bridle is just right, or its angle is too great or too small. If the kite simply won't take off, the angle is too small. Shorten the nose segment or lengthen the tail segment. If the kite shoots up but doesn't pull on the line and is unstable, the angle is too great. Lengthen the nose segment or shorten the tail segment.
Finally, it is important to make the overall length of the bridle appropriate. How far from the kite is the tow point? For diamonds, I think it should be as far from the face of the kite as possible while not being long enough to get hooked around the corner of the kite. You can experiment. It will affect how the kite flies.
TIP on making a bridle that is easy to adjust: Make the legs of the bridle out of a single length of string. Tie one end to the bridle attachment point near the nose (where the spine intersects the bow). Tie the other end to the attachment point near the tail (to the spine, several centimeters from the tail). Then tie another piece of string into a loop. Attach it to the bridle with a lark's head or prussik knot. This loop will slide along the bridle legs when you work it with your hand, but will lock into place when you pull it away from the kite. Tie the flying line to this loop.