The Map Editor already comes with the Quest Creator.
If you want to create maps without the Quest Creator, download this standalone Map Creator.
Extract and follow the instruction in the README.
Make sure the grid is set as visible in Tiled. (Ctrl-G)
You can change size via Map -> Resize Map.Recommended quest map size: 50x50 up to 80x80. Overworld maps must be 80x80.
Place stairs on that layer as well.
Grounds with different height must be different.
Grounds with same height must be the same.
Make sure non-walkable decorations will not get in the way during battle.