How can i prepare a steel planter for planting?

If it's very large, you could set it on pebbles or rocks and put the plant in another container, like a clay pot, that fits inside (you don't want the roots of your plant to sit in water. You'll get root-rot). Try this or drill holes, or just be VERY careful about your watering--make sure it is very dry before watering again. I don't know how large the container is, or what plant will grow in there, so it is difficult to tell you exactly.
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self watering planters