Here is how you make a slingshot capable of denting a metal stopsign:

Step 1: Materials

Surgical tubing; the big mamma of slingshot elastics ($17 for 10 feet at Lowe's.)

A knife; pretty obvious why I have this

An even, Y shaped stick; DUH

Leather pouch; if u want a ghetto slingshot, layered duct tape works

Cotton string; nothing to say about that

Step 2: Stick

I cut my stick with a pair of hedge clippers. I recommend hickory. The shorter the two prongs are, the easier it will be to pull back the slingshot.

Step 3: Sring

Cut 4 long pieces of string

Step 4: Tubing

Cut 2 pieces of tubing, each about a foot long. Cut vertical slits on both sides of your pouch. Run the tubing through the pouch like so.

Step 5: Finishing Bands

Double the tubing back on itself and tie it down with string. The short part of the tubing should face outwards.

Step 6: Attaching Bands

Double the ends of the bands over the end of the stick. Making sure that both sides are of equal length, tie the bands to the stick with string.

Step 7: Done!

You now have a very powerful, accurate, and reliable slingshot. You may want to experiment with different lengths of tubing and sticks.
I hope this was helpful. Comment, and tell me if it was!