A sigil is a run or symbol you create and charge with energy, to serve a specific purpose. You can use letters, numbers, even a painted/drawn image that represents your intention, and charge it for use. There are many methods of making one. Here are some links explaining some different methods of sigil creation
I created one a few years back, on my paint program. A chaos flower with letters representing my intent. The letters were too small to be seen by the naked eye. It worked wonders, repelling individuals that meant to cause problems and making them forget me lol.