It really isn't a very difficult thing to do. It does take a good amount of time if you want to make it perfect (but that isn't really necessary).
One thing that I did that was a big help was to use dymo printer labels. (They are basically thermal stickers) Set the laser to the lowest you can get it and still make a mark and just stick the labels onto the lens holder frame and tap the fire button to make a black mark.
I thought the hardest part was actually tightening the nuts after I got it all lined up without adjusting the mirrors even further.
Back to the part about not having to make it absolutely perfect. Remember that the lens has a designated focal point. As long as the laser is perpendicular to the lens face then it has to pass through the focal point. (I included a simple over exaggerated drawing)
- Some simple physics...
I would still try to get it as close as you can but for example if you are + or - 1mm from dead center with a 50mm focal length lens it will only be around 1 degree off (I would say that your gantry head has a worse tolerance).