You now have 90 chips from your initial $20 buy-in, so you are doing well. But there is one final step that can improve your winnings even more. Now we are going to bet straight up bets, the highest paying bet on the Roulette wheel. So for this you need to break your 90 chips down into 22 stacks of 4 chips (total 88 chips) and you will have 2 left over for insurance.

On an American wheel, there are 38 spots: numbers 1-36, plus 0 and 00.  Your odds of winning a one-number bet are 37 to 1 (37 ways to lose, 1 way to win).  But if you win, the casino doesn't pay you 37 to 1, they pay you less: 35 to 1.  The difference between the true odds and what they actually pay you is 2/38, or 5.26%.  You can do this same calculation for the other bets, and it comes out the same.