How do I make a planter out of a tree stump?

Maybe this Hometalk project grouping will help you:

https://www.hometalk.com/diy/outdoor/garden/11-pictures-of-crazy-cool-uses-for-tree-stumps-9621404

You could remove the inside of the trunk and plant in it with either a potted plant or put dirt in it, remember to make drain holes in the trunk so your new planter does not flood the plant.

I would just level it off and place a planter over top. The other way is to dig out the center and put in some good potting soil and whatever plants y ou prefer.

I did just that recently, I dug out the center of the stump with a knife then added a little soil and dropped in my gerbera daisy. My flower looks very happy ;)

Make sure ti dries out really well and set pots on top.