It's a small silver tool that ships with all of the machines. If you didn't receive it, you can email Sherri at
ThatsScrap@aol.com and she will get it out to you.
Before this tool that is so wonderful, we used to use 2 cds ( I think it was 2 - it's been a while!) or "eye" it. It's about 2 mm. That varies depending on how far your blade is sticking out. I am basing it on mine and tried to measure from the cutting strip to the bottom of the blade (hard angle to get!!) You want it to be low enough to cut the material when it moves down, but not so low that it drags when moving from spot to spot.
Always use the test button to be sure you are getting a clean cut and that will help you not ruin the mat, the blade , or the media.