First, I go to "Block Editor" and select the default "Palette 0". Then, I select "Picture" and "Save Tileset 1". Next, I open it up in Paint. I also open up a separate Paint page and load up the new tree I want to insert. I recolor the custom tree tile to fit the colors in the palette then I replace the old tree with the new one. I save it and open up A-Map. Then, I go to "Block Editor" and "Load Tileset 1". I load my tileset and my new tile is inserted.
So that all works out great, but then I try to do the same thing with a different tileset, and it basically overwrites what I did to the first tileset.
I do the same process for both tilesets.