Making a glass tank is not that hard, but first look at the total cost of the glass cut to size, silicon etc. Then price a new tank.
It was cheaper for me to have a tank made by a tank builder than just the cost of the glass and having it cut.
Now if your going to
DIY the tank.
The front, back and sides all sit on the bottom glass ( base ).
1, lay the base on a flat surface with some polystyrene under the glass to support it.
2, Put a small bead of silicone on the bottom edge of the glass for the back and put it in position & support ( extra pair of hands help here )
3, The 2 sides.
Put a small bead of silicone on the bottom edge and on the edge that will make contact with the back glass, put it in position & support.
4, Front glass, Put a small bead of silicone on the bottom edge and on the edges of the side glasses and put it in position & support.
5, This is the tricky bit, you need to run a nice bead inside the tank on every joint and smooth it out