Use the arrow keys to choose a tile to edit/cut. Press ENTER or SPACE to open the tile editor/cutter.

Press ESC to go back to the previous menu.

You can also use the mouse here. Click on a tile to edit it. Isn't it a shame there isn't more mouse support in the OHRRPGCE? Yep. A cryin' shame!

Press CTRL+C to copy a tile.
Press CTRL+V to paste a copied tile.
Press CTRL+T to transparently paste a copied tile onto another: colour 0 is treated as transparency.