This is from personal experience because I messed up and hated myself.
1. Arrange your rocks first before adding anything else. This will help you decide which rocks will go where because you need to know which rocks will be on the bottom. I suggest taking pictures to remember where everything goes.
2. You should add the bottom rocks in your tank first because if you add sand and then put rocks on top of it, any creature that disturbs the sand can make your whole aquascaping collapse.
3. After you have decided on the rocks you want to put on the bottom, leave them in place and pour sand around the rocks.
4. After all of your sand is in, put a bag on top of the sand, I used the bag the sand came in and it worked perfectly.
5. Put a low and wide bowl on the bag.
6. When you start pouring water in, you want to pour it into the bowl. This will help decrease the water's current and lower the amount of sand stirring up to prevent as much cloudy water as possible. You want to pour the water into the bowl and when the water spills over the bowl, it lands on the bag. Having water flow from the bowl onto the sand will stir it up also.
7. Wait a few days for your water to become less cloudy and add the rest of your rocks
Some more advice on adding sand and water. I have a 36g and its recommended about 1lb of sand per gallon, but when I added 20lbs, it seemed pretty high so I did not add the other 16lbs. After a few days the sand began to sink down and I had to add the other 16lbs which clouded my water completely and also covered my rocks with sand. Took me a week to blow the sand off the rocks and wait for the water to clear up enough to start blowing more sand off.