It might sound backward, but you are going to hang the string first for this garland. It is easier to assemble the garland while it is hanging rather than flat on a table. So cut a length of string to fill your space. Mount adhesive hooks on the wall; tie on string.
It is time to go crazy imagining monster faces. Plan on mixing colors and shapes. Maybe some of your monsters won't have mouths or will have three eyes. They're your monsters, so they can look however you want! With your plan in mind, cut out the mouths and teeth from adhesive-backed felt.