You want ot mix it together when adding it to the tank. Put a layer of Sphagnum moss over the top of it and then leaf litter on top of that. At least that is the accepted standard for substrate layers. The ABG mix is amazing for plants. You're going to love it. Assuming you already have it but you will want some Leca (Expanded Clay balls) for a drainage layer under the ABG mix so it doesn't get swamped.

I added Springtails to my ABG mix on my last tank. I think they should be added as a standard as well. Its been cool seeing them come out on top of the leaves to eat the poop and decaying leave and plant discards!