Summarize this article in one sentence.
Run the sand under water in a pillowcase or ultra-fine mesh container until the water comes out clean. Pour the sand gently into the bottom of the aquarium so it creates a medium-thick layer at the bottom.  The sand will filter water, provide nutrients to plants, and create a place for bottom feeding animals to dig and bury themselves.  Avoid using gravel, as it contains no nutrients needed for live plants. Gravel is a common choice for aquariums that don't have live plants as it is easy to clean and easy to handle. However, for live plants, gravel provides no nutrition and colored gravel can actually make the water more acidic.  If you must use gravel, opt for fine gravel over chunks of gravel, as you can mix it with sand or another substrate to provide some level of nutrition. This is not advised if you plan on having more than a few plants in the aquarium. There are a variety of substrates available at all pet and aquarium stores, and it doesn't really matter which brand you choose as long as it contains no gravel. Look for products that are clay-based to mix with the sand and look for products that are advertised as being live-plant-friendly.  Clay-based substrate will make it easier to anchor and root your plants, while other substrates may provide additional nutrition. Look for substrates that contain plant fertilizer, or mix the substrate with plant fertilizer before you add any plants to the aquarium. The sheet should cover the majority of the substrate layer as you fill the aquarium so you don't disturb it. Use a long piece of plastic wrap, a plastic tub cover, or a pot lid for spherical aquariums. The water should be anywhere from 70–80 °F (21–27 °C) to create the right conditions for most underwater vegetation. Slowly and carefully fill the tank with water,  being sure to avoid disturbing the substrate layer too much.  Your plastic cover or lid will help spread out the impact from having water poured over the base layer. Check the temperature requirements for both your aquatic animal and your live plants. 70–80 °F (21–27 °C) is recommended for most fauna and fish, but if your plants require it to be higher or lower, use whichever temperature is recommended.

Summary:
Clean and lay sand 3 cm (1.2 in) thick on the tank's bottom. Pour a 3 cm (1.2 in) layer of plant substrate over the sand carefully. Cover the substrate layer with a plastic sheet or an easily removable cover. Fill the tank with room temperature water slowly and carefully.