The Laser mirror mount has a place to hold the mirror. It has mounting holes for M6 screws for mounting to the actual laser. There are two places you can mount. You can mount in different directions for clearance. The mounting screws are also M6 size.
To insert the mirror on the 20mm mount: unscrew the circular insert. On the back of that insert you will have a nut that sets the position of the laser mirror. Remove the nut and notice there is a lip at the end that holds the mirror. Take the mirror and drop it into the area, then take the set nut and fasten it back in. You can use the two holes to get a tighter hold on the mirror.
The 25mm mount is very similar. You may notice a groove instead of the little holes. These mounts have the ability to be angled from the frame since you are mounting it using one frame, while the 2nd frame can be adjusted using the screws shown in the video.
The video will illustrate the techniques for adjusting the angle of the mirror. There will also be instructions on how to angle the beam in different directions. You will tighten and loosen smaller screws located on the base to adjust the angle and overall position of the mirror.