First of all if you never built a board you will probably have to build a couple of boards before you are satisfied. If you are going to build boards to save money then it is cheaper to buy new boards. I am now making boards for aprox 200 usd ,using only the best materials. If you will do it for fun I would say go for it. if not - kite instead.
My first boards did not work very well. I have changed materials and have been testing them hard . now I cant damage them anymore. and most important i can make them the way i want.
kiteboardbuilder.com is a nice guide, but you need to find out how it works for you, that means making boards and fail.
+ Make a model of thinn plywood, use much time on this to make it right. draw down fin positions, and straps etc.
+ You need a rocker table. This is important on boards upwind ability and speed
+ shape the foam and inlay inserts and fin plates. glue the rail on( if using plastic rails)
+ Laminate bottom,(advanced method is vacum)(easy is pressure) cut excess carbon
+ Laminate top, sand careful, cut excess carbon
+ use your model of thinn plywood to find the inserts and fin positions
And by the way, my 120/38 is my favourite board, using it in all cond.