I have written a short section about adding a laser to the shapeoko in the ebook if you want to check the basics there.
Zeroing works the same as for CNC jobs, the only thing to consider is choosing a common zero reference for the CNC part and the lasering part, for example a corner of the piece.
For engraving the surface, you won’t need a powerful laser. I went for the 2.8W Jtech, there 's a thread somewhere where I discussed why, I could try and find it. EDIT:here for example.
Adding a laser is easy enough, but please have SAFETY as your top priority. If you have specific questions on how to setup or use it, I’ll be happy to help.