switching from AZ to CO2

I had one Airzeneth for my 12 gallon tank it did fine once i filled it up, i want to switch to a CO2 bottle that i have, and i have the regulator rated up to 500 psi. I will run it at 250 tops. Running a bottle is new to me is their anything i should know abot befor i start, besides mounting the bottle at an angle?

ya i only drive my car about 3-4thousand miles a year. what size bottle did you use. how long did it last?