Start with the manual and then take a look at the video link attached. Only part of the video is pertinent for the spectral cleaning in ACL. I had some trouble with the spectral tool in ACL as it never seemed to actually work. I raised the issue with Magix and there was no resolution.
There is a better tool in the original Video Sound Cleaning Lab that resembles the one in the video. Unfortunately, it is not in ACL and Magix removed it from the current version of Video Sound Cleaning Lab. I complained to Magix.
See page 149 in the ACL pdf manual. Here is a screen shot of the page: