To put a substrate in an aquarium with water, follow these simple steps. First, thoroughly rinse the substrate to remove any dust and debris. Next, create a gentle slope in the aquarium by adding more substrate at the back than in the front. This will provide a natural look and help with water circulation. Slowly pour the substrate into the tank, being careful not to disturb any live plants or fish. Use a plate or your hand to buffer the impact of the water. Once the substrate is in the tank, use your hands or a tool to smooth and level it across the bottom. Finally, fill the tank with water slowly, pouring it onto a dish or another object to avoid displacing the substrate.