Just a few thoughts before the LR arrives. A few important rules about setting up the LR in your tank.
1) Be very careful not to drop the rock on the bottom glass. Examine the rock. If it appears to have a side with coralline algae and one side that doesn't, that side was probably 'sun-up' and should be placed that way, color up.
2) Keep the rock open so there is good flow between crevices. Caves, overhangs, etc can provide shelter for fish and are cool. Avoid the classic ROCK CLUMP. Be creative.
3) Make sure the rock is stable. Some people use aquarium safe silicone or epoxies to hold rocks together. Some drill holes and use all plastic ties to hold it together (the plastic will become encrusted with coralline algae
4) Leave room for additions in the future, eg, if you want to add a coral that is attached to a piece of rock, or, a 'frag', you may want to leave a spot or area open for later placement.
5) Leave room around the periphery for maintenance, ie, should you need to scrape algae off the glass
Some people add their sand first, then the rock. This may risk shifting of the rock if the sand shifts or if burrowing animals disturb the bed underneath it. Putting a plate or dish over the sand may prevent disturbing the bed.
The best way I think is to add the rock then add the sand. Since the cube will have seawater circulating at a high level, near the grate,( if you premixed it before in the tank), make sure you remove about 1/3rd the water so it doesn't overflow. You can gently put the sand in by cups. Every tank will have some sort of clouding that should settle. Refill with SW.
Some people have used the following:
a) electrical eggcrate--they claim it distributes the weight of the LR on the bottom of the tank. IMO, I think it creates dead areas by preventing movement and circulation.
b ) PVC tubing, 1/2", shaped into small square frames--supposedly, by placing the LR around this frame, it reduces the amount necessary by creating a scaffold with a hollow interior. Personally, I think this is better on larger tanks and having more LR provides better filtration.