How do I assemble my internal filter
It's pretty basic, though I've not seen one of this model up close, they're all pretty similar. The motor, if it's inside the box, will fit at the bottom, and the cord usually runs up the inside. Typically there's a guide channel or something to keep in one corner.
If the motor isn't inside the box, it's on the bottom outside and the cord runs up the outside. The filter media fit into slots inside the box. If it takes one cartridge, just slide it down into the slots so it sits below the top edge. If there are two slots, it can take two media cartridges, one behind the other. One may be carbon and one may be some sort of polyester stuff, but it won't really matter which goes where, so long as they fit into the slot.
And you don't HAVE to use the pre-made cartridges if you'd rather not. It's quite possible to use bulk media and it's cheaper.
I'd go with a small bag of ceramic media, placed on the bottom of the filter box firest. Then I'd place a folded wad of polyester floss on top. Just make sure the floss is not packed in too tightly and that it does not rise over the top edge of the filter.
You can also buy cut to fit filter pads; some with carbon infused into them, some plain, some with nitrate removers infused into them. Cut to the right size and slip into the box vertically.
You also do not NEED carbon. Carbon is very useful but not necessary for daily use. Best used to remove certain medications, bad smells or discolouration from water. If none of these things are problems, the space you'd give to carbon is better used for either more ceramic or more floss. To clean, simply rinse the bag of ceramic pieces in old tank water ( some you took out for a water change), and squeeze out the floss or pads in old tank water about once every two or three weeks, or whenever you notice the filter is slowing down or the media is pushing upward in the box. That means it is getting clogged up.
I find bulk floss is much cheaper, and ceramic will last almost indefinitely, so they're a much better buy overall than the cartridges or pads.
Hope that helps some.