If you have identified that air is escaping from your valve (My board is leaking but I can't find where?) rest assured that this should be easy to solve.
There are two reasons why this may be happening:
a) The valve is removable and it has worked a little loose through use.
b) There is something lodged inside your valve, preventing it from closing fully.
a) If the air appears to be coming from the outer part of the valve, you just need to tighten the valve using the valve tool supplied in the repair kit. The tightening techniques below can be helpful:
With the board deflated, grab the valve basket through the underside of the board with one hand and use the other hand to tighten. Alternatively, you can place the board on the floor, and kneel either side of the fin box to pull the PVC taught (this will create pressure and push the valve basket onto the floor to help hold this still). The valve can then be tightened.
Here is a video where Alex talks you through the process:
b) If the air appears to be coming from the inside of the valve, around the dust cap, It is likely there is some debris inside the valve which needs to be removed. This can be done in one of two ways:
Tap down on the centre pin firmly and quickly a few times to dislodge the possible sand/debris. Be careful to look the other way and keep your face away from the airstream, preferably wearing glasses/goggles.
If this doesn't work, you may need to remove the valve. The process is similar to the one detailed above, but here is another video of Luke explaining how to do it properly: