When choosing the position for your rainwater tank consider getting access to it - you will need to get the water out of the tank, run the overflow to a suitable place, and get to the tank for maintainence.
Each Waterwall Fatboy you are installing requires a level area 8' long by 32" wide. Tanks joined with the Joiner Kit are placed hard against each other, end to end, so the base required is 8' long by 32" wide for each tank.
Pour a concrete slab to the required size which is at least 4" thick.
Install concrete pavers which are at least 2" thick along the length and depth of the area. Ensure the soil underneath is compacted, and that there are no gaps between the pavers. Make sure the pavers are level from front to back, and from end to end.
Using external grade hardwood, prepare a solid wooden frame to the desired size. Fill with 4" of builders sand or finely crushed rock. Compact the base material and make sure the surface is level from front to back, and from end to end.