Hello & Welcome

Using custom tilesets are easy.
If you have some tilesets (each custom tileset is a bunch of custom graphics + *.tileset.ini file which a settings of custom tilesets) put them together with your level file (or into its custom folder), and when you open the editor, you should see them in the tileset item box's 'custom' tab (View -> Tileset item box).
If you wish to create a new one, simply press the "New tileset" button to open the tileset editor
And don't forget about the "Level specific" checkbox which will save your tileset settings into the custom folder of your level / world map.