The pack on the left will serve as the base of your new mixed pack.
What you see here will be what you get in your game.
The pack on the right is the one from which you wish to pull tiles.
To replace a tile, first select the tile you wish to replace in the base pack, then select the tile you want to replace it with in the donor pack. It's that simple.
Once you are done with your mad mixology, click the Save button and pick a filename for your new pack.
The Reset button will remove all changes you make so be sure you want to do this before clicking it.