Cut, stack or fold the garbage that needs to be picked up. Be mindful of local regulations about the maximum length and weight of these bundles, as you work.
Use the string or rope to tie the bundle together. To secure the bundle, tie it around both the front and back, and left to right, so that you have one intersection covering four directions. The specific knot you use won't matter, just make sure it is secure.
Take the bundle, along with the rest of your trash, to the curb for pick-up. If you have followed local rules, then your garbage men will take the bundle away with no problem.