I began with the large planter and rock work of the back wall, because it was the largest and most complex of the pieces to be constructed.
Lay the aquarium or terrarium on its side to make it easier to work with.
Because the final design will be incredibly heavy, you need to make sure the rock work is removable one piece at a time. While working with the cement, always tape a lining of waxed paper or saran wrap firmly in place, so it doesn't move around or fall down a lot and drive you crazy. This lining will be removed after the piece is completed and will ensure that the cement doesn't bind itself into your terrarium permanently. It may seem like a hassle, but if you don't use a lining, the cement WILL stick to the glass.
For specific instructions on mixing the cement, look at this instructable: https://www.instructables.com/id/How-to-mix-hydraulic-cement-for-sculpting/
Mix the hydraulic cement a little at a time and build up what will be the back wall until it is the shape you want. Smooth new cement into the existing pieces to prevent gaps and leakage. Add new cement while the prior pieces are hardened enough to keep their shape, but still damp. This is important for strength and to prevent leaking later on.
I was able to make the back wall very thin for this planter, because I later added horizontal ledges and ramps, which added to the structural strength. The fewer ledges and ramps you are planning to add, the thicker you should make the back wall.
Build up the bottom of the planter, making sure to seal the bend very well. Make the bottom as robust as possible.
Cut the small plastic or peat planters and place them where you like them, then cement them in, again working in small amounts of cement. Leave at least one drainage hole unobstructed at the bottom of the small planters.
This step of building up the basic shape of the back wall and cementing in the planters took approximately three hours for me.