Outs * 2 if there is 1 card left to come.
Outs * 4 if there is 2 cards left to come.

Generally you use the * 2 method unless you are calling an allin on the flop, as you may face another bet on the turn, so you may not get to see both cards for the price you are being offered.

That will give you the percentage your hand will hit it's outs. But not the percentage your hand will always win. Since we don't know our opponents exact hand, there will be times when our out comes but we still lose.

To convert the % back to a ratio, divide the percentage into 100 and subtract 1 from your answer. That is if you have a 20% chance of hitting your hand, it's 100/20 = 5... 5 - 1 = 4.... So you have a 4:1 chance of hitting.

Hope that helps.