There is a lot of information about how to properly fill a planter. However, we recommend the following process:
1.) Make sure the planter is level and is elevated above the area it is sitting upon to prevent condensation.
2.) Add a filler that is not biodegradable, if the planter is considerably larger than the space needed for the plant to grow.
3.) Place landscape fabric to separate the filler from the soil and allow excess water from a possible over-saturated soil to leave the roots.
4.) Add about 6 inches of soil. Make sure the soil you use is appropriate for the plant. Then, tamp down, add more soil, and repeat.
5.) Add Plant sipper, if desired.
6.) Add the plant, more soil, and then wet the soil.
7.) Leave about an inch space between the top of the soil and the top of the planter
Learn more at https://www.puremodern.com/blogs/news/how-to-properly-fill-a-planter