IME, Working with large rocks makes it much easier to create caves and overhangs. Long rocks are invaluable when aquascaping.
Try to place sturdy rocks with flat but small bottoms as the base.. and jiggle as you go
Test each teir with a firm jiggle to be sure it is stable and will not tumble easily. Try to cover as little of the substrate/bottom as possible yet allow for steady support.
If you don't use a substrate, invest in a protective layer of cutting board along the bottom to prevent glass breakage during an avalanche.
If you do use a substrate, placing the bottom rocks on the glass (under the sand) or using a PVC stand to support the rocks. This is most important if you ever add any sand burrowing critters that might dig tunnels through the sand.