You can do this two ways, one would be a sump and the other would be a refugium (with a bigger sump tank you can combine both).
For the sump you would house your skimmer, heater, and return pump. You need a HOB overflow that will take water from your display tank and send it to the sump beneath the tank, your skimmer and heater would go in the section the water falls into, then you want three baffles before your return pump so the bubbles don't get into your tank.
I think for such a small tank a better idea might be a refugium this can go beside or below the tank and has pretty much the same idea, but instead of baffles you want a seperate compartment for your return pump, the rest of the tank has a sandbed, LR and macro algaes.