The first thing you'll need is the large-format camera for which you want to make a lensboard. Actually, the ideal situation is that you not only have the camera for test fitting, but also one workable lensboard for it that can serve as a reference. Many large-format cameras have somewhat standardized 4"x4" or 6"x6" lensboards -- all the examples here are for my B&J 4x5, which uses a 4"x4" lensboard.
The next thing you need is an understanding of which of the three types of lensboard is appropriate for the lens(es) you want to use. The photo shows examples of all three types: barrel lens with mounting flange, barrel or shutter mount with rear locking ring, and generic (E-mount) bayonet flange. This is roughly in order of increasing build difficulty, although none is all that hard to make. Step 3 only applies to thick lensboards that must have thin edges. Each of Steps 6-8 is specific to one of the three types of lensboard shown in the photo here.
So, what else will you need? Read the complete Instructable before building anything, but here's a rough list:
Material for the board; probably scant board (you can get various nicer woods for a few dollars via the Internet)
A saw, etc., and sandpaper to cut and finish the board to size
An adjustable hole saw or other device that can make a hole in the board
Paint and brush or other finishing materials
Only if you're building the third type of lensboard, you'll additionally need:
A cheap extension tube set -- the kind with front, back, and three screw-threaded in-between segments
Glue for setting the extension tube in the board hole
The total cost is quite small -- probably less than $10 even including the extension tube parts. It can take a couple of hours to make one lensboard... or a half dozen.