With a heated soldering iron, slowly make a hole through one of the T pieces on the side that is above the larger pipes. Make sure that soldering iron reaches the lower pipes, so you can mark them for later.
On the pipe that we have just punctured, slowly widen the hole with circular movements, so that silicon tube used for aquariums can barely fit. This part is very important, because if you make a larger hole, you will face problems in the next step.
After widening the hole, use any brand of waterproof instant adhesive (in our case, we used Loctite SuperGlue) on one end of the silicon tube.
Next, place the tube inside the hole so that it only reaches the inner layer of the pipe.
This is where the water will flow from one set of pipes to the other.