Hi bro, typically
1st compartment
when your water from the overflow enters the sump, it should go through the mechanical filtration 1st.
Hence cotton filters (I wold recommend filter socks) should be the 1st step.
You can then put the chemical filtration stage next, which are the carbon, polyfilter here. You can also put the phosphate removers here if you do not have a FR.
Last comes the biohommes/ LR in the lowest part of your last compartment. This is your bilogical filtration.
2nd Compartment
You can place your skimmer here. (you might want to put some cotton on the water overflowing to the 3rd compartment to remove any microbubbles caused by your skimmer.
3rd compartment
Return pump