I hope I'm not late in telling you this if you are preparing a cemetery urn but when planting a planter or pot you must use potting soil and not your backyard garden dirt.
In the garden centers you will see "Top Soil" and "Soil less Potting Mix". Get the soil less potting mix. Why? Topsoil, without earthworms, will set up almost as hard as concrete in a very short period of time. The soil less mix will always stay soft and light, allowing the roots to maneuver and spread out.
Plant selection is important too. If your planter is to be in the shade you will need shade loving plants; if in the sun all day shade loving Impatients will cook.
Water according to weight. Lift it to remember what it weighs when dry, then water it and remember its weight. From then on let it approach its dry weight for very short periods of time so it can breathe, but try and keep it moist most of the time.