This is repeating the essence of the instructions in the project description.

Cut a 50mm piece off the end of the piece of wood, and in the 50mm piece machine out a T-shaped slot to hold the coiled part of the spring with the screw through the middle of it. See pictures for a close-up. I used a Dremel for this, you could just as easily use a small chisel or even a knife.

With the spring and screw in the slot, screw the 50mm piece onto the remainder of the wood such that it traps spring and screw. I drilled a couple of guide holes to take the screws in mine, you may choose not to.

You should now have a completed spring clamp.