I'll give you links to the tutorials for DIY 70W HQI MH, the same can be used for 150W MH, only reflector unit should be rated 500W to be suitable for 150W bulb by size, and ballast should be for 150W.
Or you may do search, using these terms, only changing wattage.
1,
2,
3.
I did that, it works even with different brand of reflector unit.
The bad part:
Not much info on where mount the ballast.
It's heavy, and I wouldn't like to have this additional weight resting on the top of tank, and I prefer an easy access for feeding and cleaning. As I understand it, when turn it on, the short very high voltage impulse starts - so the wiring between ballast and the bulb should be reasonably short.
When pedant is not an option (suspended ceilings), and the unit is not near the wall (to make a wooden support behind the tank), then there is a problem. Mine is free-standing unit near big window
