In Spain, slingshots have a long tradition. They have a very good way to attach the rubber to the fork, and the slingshots are outstanding masterpieces. eltirachinas asturiano shows some very good examples.
First, think about the shape the slinghshot should have. Draw it on cardboard or plywood, so you get a template which can be used over and over again.
Take a piece of wood, about 2 cm thick. It has to be hardwood, like beech or olive wood. A beautiful wood makes a beautiful slignshot! Use your template to draw the shape onto the wood and saw it out. Then, grind away the sharp edges, make the handle comfortable, paint it if you want etc...
...and drill a hole into each fork end, 1-1.5 cm deep and 5 mm in diameter.
Ok, you're done with the woodwork. Now, you need rubber
string(waxed string is best)
two strips of strong leather, 5mm thick and 8cm long
a piece of leather for the pouch, about 2,5cm x 10 cm (can vary)
Now, put some glue into the holes and push the leather strips in ("screw"them in, that works best). Then, force as many tips of the toothpicks into the hole as you can. You must use plenty on them in order to make a secure slingshot!
Make holes into the leather pouch and connect it to the bands. This works best if one person pulls the rubber straight and another person makes tight knots around.
That's it! You've got a slingshot. Have fun....
Wait until the glue has dried and attach the bands to the fork in the same way.
And be careful not to shoot an eye out!