Ten easy steps to making a cave interior if you've never made one before.
1. Make a new map. Make it 50 x 50.
2. Now go to Generate Dungeon. Like I said this is the easy way! Pick the tiles you want to use.
--- I am going to make an ice cave. Why? I haven't made one in a year.
3. Looks like crap doesn't it!!! Pick the point of entry and use your floor tile to make the entrance to the cave.
4. Use a tile to make a rift through the map. I mean use lava, water, a pit, or something to just cut through your map. Cut through it wisely.
--- I chose water, because I do have an ice cave, and again, it still looks like crap!
5. Touchup what you did. Just cutting through the map like this ended up making features look ugly. So use the cave wall to touch it up or change certain ceiling tiles to floor tiles.
--- Move and manipulate the path if you want!
6. Nooks and crannies! Add mini paths and manipulate the walls so they are not square. Unless it is a man made dungeon, caves are not square.
--- To make this easier, highlight 4 tiles, a ceiling tile, a floor tile, and two wall tiles between it. Then use that to carve out your walls.
7. Add a variety of floor and wall tiles! Add dirt, sand, grass and other floor tiles to spice up the floor. Add different wall tiles to spice up the walls.
--- Is your cave ONLY a mine? Or did the miners break through to a secret lava tunnel? Is your cave only an ice cave or is there a frosty cold dungeon in it? This will give your cave some variety so it feels like you are actually going somewhere instead of getting lost.
8. Detail the walls! Start with waterfalls and cracks in the wall. Where is your lava or water coming from? Waterfalls, lava falls, or cracks that's where! Then add cracks, vines, plants and anything else that could go on a wall.
--- Make sure each area within about 2 tiles has something there. Something different. Use pillars, cracks, plants, tablets, torches, chipped away rock or anything else you can think of.
9. Add details to the floor. Add rocks, fragments, stelagmites, statues, crystals and make sure there are interesting paths to take not just straight lines! Take this time to add the bridges too.
--- Theme the rooms even. One room can be a dungeon. Another room can be the dining room to eat flesh of mortals. Another can be a throne room, and still there can be a magic room or a storage area.
--- Don't just use birdges, maybe jumps, moving platforms, ropes and so on.
10. Add the events like monsters, treasures, random things, glowing torches, fires and so on.
--- If you want to make a cave with multiple floors, just add staircases and create a new map and start from 1 again.