Shape the roast into a cylinder (fatty side facing upwards) while tucking any loose pieces in and under the roast. Try to keep the roast uniform in size from one end to the other so it will bake evenly. Use individual pieces of twine to tie around the roast at 2-inch intervals. Tighten the twine just enough so it presses firmly against the meat but is not cutting into it.
Alternatively, you can cut a piece of kitchen twine about ten times the length of the roast. Loop the string around one end of the roast (so it presses firmly against the meat without cutting into it), about 2 inches in from the end. Run the string down 2 inches along the top of the roast and then wrap it around the roast, making a loop. Pass the string under the loop and gently pull, tightening the loop around the roast. Move down the roast, making three additional loops at about 2-inch intervals, and tighten each one so that the string is pressing firmly against the meat. Flip the roast and pass the string in and out through the loops along the bottom until you are back at the top where you made the first loop. Tie these off together.