Not much to it, just cut a chunk of seatbelt or other webbing as shown here,
cut a carpet scrap, and bolt it to your kiteboard as shown here.
These straps felt really good on my feet.
I was kiting in Australia and didn't have my plywood board. Fortunately a friend had one, but without straps. A few minutes later, problem solved, and I got to ride a great plywood board. A good day.
The bolts for each strap should be inches wider than your foot. Don't toe your straps too far out. If there's any way to trap one foot and release the other, you've just made a joint-breaking machine. A friend just wrecked his knee this way and went crazy while the rest of us had many good days on the water. The straps need to be wide enough that you can pull your foot out even when twisted sideways as far as it can go. For size ten feet in booties that means at least 7.5 inches. The straps need to be inelastic and snug enough that your foot can't get jammed too far in. Some people like wooden block standoffs under the ends of the straps to raise them up a little. Others don't.
Step 1: Ugly Leash
Most people don't use a leash anymore because of natural selection.
Recent kites have a good lift-to-drag ratio and people body drag back to their board when they drop it.
Everyone seems to have a story about almost getting killed due to a board leash.
On this particular day the wind was fluky and I wanted a leash. Here it is, thanks to a lobsterman whose potwarp washed ashore.
The knot is NOT A SLIPKNOT.
It's a sheet bend, figure 8, or other boyscout knot. Slide it up your leg til it stays. If your board gets eaten by a monster or dives into the sand like an anchor maybe you can wiggle out of the loop.
Still not safe, but more convenient than struggling back to the board in wrong conditions.
Step 2: Plywood Board
While we're at it, here's the plywood board.
Use whatever plywood you have. No thicker than 1/2". Face veneer grain running the long way.
Your first board should be 16" wide and 60" long.
If you have wind over 15 mph make the board 14" or 15" wide.
Rectangular is fine, feel free to round the top or bottom edge and make the outside shape whatever you want. Put fins on them if you want, but not too big or too sharp.
Everything works, they all feel different.
For more details see the Kiteboard Cookbook.
To make your own foot pads a.k.a. stomp pads, see Make Your Own Kiteboard Footpads