this was taken from the link you gave ...
Skimmer Installation:
The skimmer can be placed either directly into an aquarium or into a small filtration part of the aquarium. Please note that the skimmer works best inside a filter box as further filtration can be used to reduce micro bubbles.
After choosing a location for the skimmer, attach the pump to the skimmer. Two 90 degree elbows are included to allow the pump to be mounted in multiple locations for better fitting.
Secure pump and skimmer securely into tank using mounting suction cups provided. As an alternative, a mounting clip is also included for clipping the skimmer to internal filter walls.
Ensure that the air supply line is attached up out of the water. Air must be able to flow freely into the pump intake in order for proper operation.
as for the PH's, in a 12 gallon tank, i would go with 1 MJ1200 for now. if the need arises you can always add the other. the reason i say this is because they are really only going to be used for flow. depending on what you plan on keeping ... you may need more, but the odds of needing more flow than the filter AND the PH is not likely. with a 295gph MJ1200 yoyu will be turning the tank over at 24.583 times per hour, that is pretty good. but as i said before, it really depends on what you will be keeping in the tank.
I am excited about setting it up, i'm going to start purchasing the pieces this week. Probably the tank, PH and skimmer first, then continue on from there.