Smoking the 'weed' is perfectly acceptable, and even encouraged during basic training, as it improves performance, gives you an edge, and turns you into a natural born, mad and up for it killer. It is considered good manners to offer a 'joint' to your training staff, and will make your passage through basic much easier.
You can smoke when you like. When your new CO stops for a chat when you are stagging on the gate, just pull out that 'nipper' from behind your ear, spark up and ask him how the wife and kids are. He'll respect you for your consideration.
Smoking is positively encouraged. Not only will it make you appear windswept and interesting it will also make you dramatically 'more funnierer' and hugely 'shaggable' It has even been considered to make you 'biggerer and cleverer' than your average 'Joe Crow'. Might I suggest a job in the RLC as perhaps an Ammunition Technician or Petroleum Operator as both of these trades are hugely fond 'smurkers'.