Trussing is an optional step.  I like to do it to help keep the legs looking nice and not falling apart- particularly when turning the bird.

Tie one leg . . .

Wrap around the other. . .

Bring it around one side snugging the leg against the body. . .

Some people prefer to bend the wings down and tuck the tip under the turkey- If a big bird is not getting flipped, so do I.

Repeat the process on the other side. . .

Tie it up, and you're ready to go.