Step 1) If you want wood or rock in the foam background and they are light like cork bark then you will want to stick them to the back of the tank with aquarium sealant. I used black aquarium sealant.
Step4) Coat foam with aquarium sealant and working in sections while the sealant is still wet cover it with dried coco fiber (press it down good). I find that the best way to apply the sealant is to squeeze the tube out onto the foam and then rub it around with your rubber gloved hands making sure to get into all the crevices. Once all the foam is covered let it dry overnight.
Alternatively if you don't care about the shape of the background you can put the coco fiber directly on the great stuff while it's wet (skipping the cutting and sealant part) then let it dry overnight. this saves a lot of time.
Step 5) Once everything is dry You can now put substrate in the tank and decorate it as you like. The foam and sealant are long lasting and inert giving you a beautiful homemade background for years.