Very simple system. Bet 1 unit on the dozen, increase 1 unit on the dozen after every spin. If it hits, and bankroll is +ve reset progression to 1 unit if bankroll is -ve continue increase 1 unit on dozen.
You may want to set aside a stop unit (if it increases too much and you can't cope it stop it on the next win)
Overall this system can make easy $$. Try it, I would be quitting roulette fron now onwards to focus on other stuff.