Get 12 dice of various shapes. Make about half of them visually distinguishable from the rest. I chose dark and light:
|Whoops, my d10 with the dark 7 should be on the other side|
Toss the dice on the paper. If some roll off the paper just move them back to the closest edge:
Now trace the dice in pencil as you remove them and write down the number they rolled. Light, sloppy, and fast is good. If several dice clump together, trace around all of them like its one lumpy room. Also put a little dot for the dark dice:
Now, clean up the connecting passages and draw little curved lines to indicate differences in elevation. I draw them like little steps leading from the lower areas to the higher, each indicating a rise of 10'. If a difference is too much, make it a sharp cliff:
One last thing. If we roll a 12 sider and just count clockwise (roughly) to pick a room, we can place a water source and see what it does. If there are lower elevations nearby, let it flow along. If not, it can be a pool, or you can disregard- not every cave has to have water.
The largest numbers are the exits to the surface. So, here the 15 and 13 have entrances from surface caves or sinkholes.
Now we have some treacherous terrain to explore: narrow passages, long drops (one 60', one 100'), and even a dry cave hidden by a nearly impassable underwater one. Next we'll place some monsters.