Before you start
Check that your plant pot has adequate drainage. You are looking for either existing holes in the bottom of your planter or removable plugs. You may want to consider placing a saucer under the planter to catch any overspills. This is particularly important if your planter will be kept inside. It is key to allow water to safely drain out of your planter to avoid your plants getting root rot. You can position your planter onto pot-feet, rubber mats or allow water to drain into a reservoir to allow water to drain effectively.
Finally, before you start filling the planter, position it in its final location. This is particularly important if you are using large planters. Once filled, the planter will become heavy and difficult to move. It will be far easier for you to begin filling the planter when it is already sitting in its intended location.