First we build the base that fits the tank. This way we can take it out of the tank and build on that instead of having to carve and assemble things in place. Cut the panels so they fit the full size of the tank, you can trim them later if you dont want it in the final structure but it serves as a guide while you're building. 

Next start carving and assemblign the backdrop texture, and building up any structural supports for your design

in our case we chose to go for a layered/stacked look so we cut strips of foam then shaped them to look a bit like rocks, dont worry you dont have to be an artist the concrete will do a lot of the making it look like a stone, just get rid of all the unnatural right angles and you'll get something rock like

Once you've got the carving done, put it back in the tank and then mark off where you want to trim it for the final design, unfortunately i don't have any pictures from that stage. but i do have a picture of a bulldog so i will include that instead. Once you've done that pull it out and cut it to its final shape. 

Im putting this all in one step because its basically the same step over and over with very minor changes. 

Step 4A, mix up the surface bonding cement as instructed, do not add any dye, fortifier or glass fiber, do it as instructed but add about 2-3% more water than instructed you wont need a lot, on my tank i only needed to do it a few cups at a time. 

Step 4B Paint the form with this concrete, regular old paint brushes, cheap ones, you'll ruin them. 

Step 4C Patience. 

Step 4D, more patience

Step 4E, once its fully dried mix up another batch of concrete, add about twice the recommended amount of the acrylic fortifier and no glass fiber. repeat B,C,D. Do this twice 

Step 4F, same as the last step but add glass fiber and about 3-4x the recommended acrylic fortifier,start adding the dye if you are using any. repeat until you are happy.

Step 4G, seriously this is a lot of steps for 1 step in a tutorial, but just do a final coat, with no glass fiber and and substantially more acrylic fortifier than you feel comfortable adding, roughly 5x concetration is what i used. do this once or twice to get desired texture. 

Now that its fully dried and awsome, its time to put it in the tank

use an obscene amount of silicone, not so much that you cant push it flat to the bottom and back, but ALOT. this will want to float pretty badly. 

Then silicone around all the edges to make sure there is no way water gets under it and cause disgusting things to happen. 

This is TEDIOUS 

concrete, is not fish friendly until its been flushed out. 

so first know your tap water (or whatever water you use for this) pH level. 

Fill the tank, add some circulation (not a filter, it will get gunked up, just a power head or whatever)

check the pH it will be insanely way off where it should be, drain the tank

fill it up again and repeat this until your pH remains stable. after 24h. then drain it one final time and prepare the tank for inhabitants as you normally would