A large shipping box is perfect for building a putting green. I used a box from a flat-pack piece of Swedish furniture, so the dimensions required no modification, just the addition of the obstacles and cup. The two halves of the box are laid end to end for a longer green. You may need to cut, fold, and join parts to construct a similar green, depending on your materials.
Place the hammer stand behind the hole as shown, so the hammer head strikes just a bit in front of the cup.
Place the striker assembly so that it will hit the hammer properly -- you can turn it on to test the best position.
Then, tape both the hammer stand and striker assembly down with packing tape.
Pull the spring up and over the striker assembly box so that it has just the right amount of tension to help the hammer up, but not so much that it never comes back down!
You can now take a strip of cardboard and make a protective back rail. This will keep the ball from falling off of the green, and from destroying the mechanism!
Just make sure you cut the strip of cardboard narrow enough so that it clears under the hammer's handle right at the pivot point.
You can use tape or glue to hold secure the rail. If you like, add more cardboard strips for additional side rails.