If he has 2 diamonds, then he has 9 outs which give him a 35% chance of making his flush with 2 cards to come. In odds, that is about 1.85 to 1.
It's very hard after the flop to make a bet without giving him the correct odds to call, because if you bet $100, which is the size of the pot, then he would be getting 2 to 1 on his money (pot would be $100 + $100 = $200), which justifies a call. So you would have to bet an amount that is actually bigger than the size of the pot, the correct amount is around $150.
Not giving the right odds usually works best and is easier after the turn, because his chances to make his hand are sensibly smaller with only 1 card to come. In the same example of a flush draw, on the turn he has 9 outs with 1 card to come, so he has a 19.5% chance of making his hand. In odds, that is 4.1 to 1. Without me having to say it, you obviously understand that the difference between 1.85 to 1 and 4.1 to 1 is enormous. If pot is $100, as in your example, a bet as small as $50 gives him 3 to 1 which makes his call incorrect. That's not to say you should bet $50, I'd bet more. The reason for that is implied odds.
The concept of implied odds is well explained in the articles section in this site. It's principle is that if you hit your hand you will squeeze some money out of your opponent, therefore you can adjust your correct odds range accordingly. I suggest you read it. There is also an article on pot odds
. I just want to add here that because of the implied odds, in the example of a flush draw after the turn, a good bet size would be 3/4 pot at least, which is a pretty standard turn bet anyway. You always want to bet an amount that will show you a profit in the long run in that situation, and a loss for your opponent consequently.
I hope this helped you a little. To complete this check the articles section as I suggested. If you have trouble understanding just ask.