First, fill your tank with tap water and run all the pumps for a day or two to check for leaks. After repairing the leaks that you will certainly have the first time, then:

Put something on the bottom of the tank to protect to the glass, set your rock, then put the sand in. That's my preference, everyone will have their own opinion. Then put the water in. It will probably be days before you can see through the water as it will be extremely cloudy at first.

Bottom: several options. There's starboard which is what cutting boards are made from, there's egg crate which is sold at hardware stores as light diffusers, and/or PVC pipe frames made by you that hold the rock in place.

Whatever you do, make sure your rock is solid in place and can't be moved around or shifted. There are some common creatures that make for very powerful bulldozers.