Here's the procedure I used to align the laser. You may find a better one.
 

• Start with slits in front of the laser wide open. Make sure the beam is going through the center of the slits and also through the beam splitter.

• Set the rotating mirror so that the "front" is facing forward (the front is shinier than the back).

• Slowly rotate the rotating mirror so that you can tell which spot is the beam and which is a reflection. Note where the beam appears on M1.

• User the beam adjuster to raise/lower the beam (parallel motion). Adjust the beam horizontally so that it goes through the center of the rotating mirror and approximately through the center of M1.

• Look for the beam on M2. Adjust M1 until the beam is centered on M2.

• Adjust M2 until the return beam is again on the center of the rotating mirror.

• At this point you may see reflections back near the laser. Forge ahead!

• Turn on the rotating mirror. Looking through the eyepiece, look for a bright spot. DO NOT look at the spot UNLESS the mirror is rotating!

• Change the rotation frequency. You should see the spot move.

• Move the eyepiece to get the spot in the center for a given frequency.