I added sand to my tank before, so I can help a bit on this. The sand MUST be washed if it is dead sand. Emphasis on MUST. There will be contaminants like fine sand you don't want in your tank. And washing the sand reduces the amount of fine dust particles in your tank, preventing it from being cloudy.
When you pour the sand into your tank, pour into one side, and chase all your fish and mobile inverts to the other side of the tank. try to get the sand to be in a fine stream, and try not to empty the sand all at once. A lot of water will get displaced and your floor would be very wet. It does not matter if the sand lands on your rocks, but be wary if it lands on corals. Some corals cannot move to brush sand off itself so you will experience coral death if you don't clean the sand of your corals. Once you have poured the sand on that side, move it (very slowly) across the tank and flatten the sand. This minimizes the old sand being upturned and sending up unwanted detritus back into the display tank. Once the sand has been flattened, for personal preference of aesthetic appeal, I try to slope the sand onto the rock to make it look like there is more rock under the glass. Also do not lift any live rock structure when you pour your sand. This will cause the structure to become unstable. The weight of the sand also prevents the rocks from toppling over a lot. Hope this helps