While you've already seen some of my pictures in the manual, I will add a short explanation on how i've approached the alignment of the mirrors.
As indicated earlier by Bonne, I made a new test fire macro in laserweb, containing the gcode 'G91 G1 Y1 S0.2 F3000 G90'. This moves the Y axis 1 mm with the laser on. This is a relative move (G91) so while the axis does slowly move further along the Y axis, you do not change the distance between your second and third mirror, and can constantly reuse the button instead of having to reverse your direction all the time. It does require you to move it back a little every now and then or you run out of adjustment window
One other important thing I did, and I suppose that deppends on if you have the same JSON with settings (from Bonne) imported as I did, is to change the move to 0,0 macro into a G0 move. The existing macro did not specify which mode it needed to use for the movement, and as the G1 is modal, it will move to 0,0 with the laser on. Needless to say, such a long move will cause fire on the stickers
Now, for the alignment, after various tries, the following worked for me:
1) Make sure the beam hits mirror 1, then that it hits mirror 2
2) Move the carriage all the way to x-min
3) Adjust mirror 1 so that the dot ends up center of the laserhead oval
4) Move the carriage all the way to x-max
5) Adjust mirror 2 so that the dot ends up on the same position as when it was at x-min (I needed to move between x-min & x-max a few times to create new measuring dots, as sometimes the sticker was so full of dots close to each other, it was hard to see what I did)
6) Put a sticker on top of the nozzle opening (on the inside, black part) and adjust mirror 3 to hit that in the center
7) Put the sticker on the outside of that opening (still without the nozzle in it) and adjust again to hit it in the center
8) Screw in the nozzle, tape a sticker to the opening, and make sure the laser hits the paper through the nozzle
9) Now if you put some material underneath the nozzle, z-probe, and put a sticker on top of that, you should still get a dot
10) Screw in the lens and burn away
Ofcourse when I put the nuts on the adjustment screws, I managed to mess up the alignment again as for me it was almost impossible to hold the screws so still that there is no movement at all in the mirrors. So for the third or fourth try, I did my adjustments and tightened the nuts at the same time. So each adjustment involved undoing the nut, adjusting, and retightening it. All in all, this trial and error took me two half days.