Yes sounds like you have it right. eg. White pawn on a2, black pawn on b4. White moves a2-a4, black captures en passent bxa3. The black pawn moves to a3 and the white pawn is captured. You must do it the very next move after the opponent makes the 2 square pawn advance if you want to do it. Sure someone can post a diagram for you.
En Passant Question

I tired googling En Passant but I feel like the explanation for it is over complicated for no reason.
Correct me if I'm wrong.
1.En Passant can only take place between pawns.
2.When you opponent decides to move his pawn next to your pawn instead of attacking you.
3.You attack like a pawn usually does(diagonally 1 space forward) and remove his pawn.
4. You have to do it right away (after 2), you can not come back to it.
Am I right? Some of the explanations say that you pawn has to move 2 spaces and his moves one. Im not sure if this is correct