Where to place skimmer in sump

[2006sandy]

Where should I place my skimmer in this 3 compartment Tsunami dual sock filter sump that I purchase for my 125 gallons saltwater reef aquarium. Please see picture. (hopefully it attached) Do I place the skimmer in the middle? or in the last compartment?

 

[xanthurum]

In the middle, the last compartment is for the return pump.

 

[2006sandy]

Thank you so much xanthurum

 

[2006sandy]

Also should I place sponge block and carbon in the red chamber? or just use a sponge block? And since I will be using a heater as well, do I attach the heater in the tank? or in my sump? I'm so sorry for all the questions, but I want to make sure I do this right since I'm new to it. Thanks a lot!