When you hear about double barrelling in poker, you just need to think that you are firing a gun twice in succession. You double barrel when you lead out the flop and also go ahead first on the turn. Basically, you double barrel when you raised before the flop, when you get a draw, or the board is quite ragged. Let us focus on the pre-flop scenario.
If you plan to double barrel after doing a raise before the flop, you need to think of the amount of chips that you have, the stack of your opponents, and your position on the poker table.
Let us say you have a small pocket pair and another player calls your raise. In case the cards reveal K-A-5 and you have 6-6, then the best move will be to bet at least half to two thirds of the pot size. You need to take down the pot before you opponent tries to outplay you with the ace on the table. Pushing a sizeable bet will demonstrate some good strength.
In case your opponent calls, then wait for the next card. If you see a face card, then go try to bluff your way to the pot since your opponent will try to play you with a straight or two pairs. Make sure you push more chips, around ¾ of the pot.
When you are in a late position, having the same picture above, then double barrelling is even more advantageous. When the competition leads out, then the move might be out of the question. When they check, then by all means get in control and push them to the edge.