First you need an entrance, If you already have some, terific.
If you need cave floors on a map where they dont exsist.. theres a couple things you can do. Find a cave on another map, thats similar size, (OSI map has loads) and melt it.. export it, and then re-import it to your cave..
Or make a new one. Its really not all that hard..
What you can do, is make a cave floor layout in green acres, size it to the interior of your mountain cave. Export that cave floor you made, and go back to your cave.
Now it gets tricky.
You need to plan this out well, Move your character with a .px and y (pol) or a .incX and Y (RunUO) commands, to get you to the SW corner of the area, place a cave tile where you are standing, and import the floor, targeting that cave floor tile you just placed..
If you mess up, wipe it, and start again..
After you get it right, Re-export the new placed floor, and use World Forge to burn it in.. your done.
Also, you can paint one on a bmp, and use Dragon to make it for you. Its the best way, since it auto generates the edge tiles. Making one by hand, in game is very tedious, I have done it several times in certain areas, and dont recomend, nor wish it on anyone
Then melt that new one, bla bla...
Unless you need to touch up your cave entrance with Map-edit.