Here's the story of how I went from an empty 90 gallon to this:
Started off by siliconing a pane of glass to the right rear corner to hide the filter intakes and support the main root.
Then I mocked up some texture pieces considering how flow in nature would scour out some of the bank.
After that I siliconed the main piece of foam to the back and attached the wood with spray foam.
I then added the texture pieces and began smoothing them out. A razor blade was used for the gross stuff and finished off with sandpaper.
I covered small areas with West Marine 2-part epoxy then completely covered it with play sand. It turned out to be too light for my liking so I did a second coat with sand mixed with peat moss. After mixing them well the moss was broken down enough to darken things and look like it was natural soil.
After that it was just a matter of covering areas and using the mixture to fill in gaps to smooth things out.
There we go. A DIY custom background. It stayed in there for about 5 years before I ended up tearing it out due to old age and the silicone starting to lift.