In 192 colour pages you are whisked through 25 types of creative map, but you begin with basic advice and components such. And here's a post that might help clarify some of the instructions in the Tome. A Creative Guide with Tips, Tricks, and Projects (2018) is the book. If you have the Tome of Ultimate Mapping, you can find instructions starting at the bottom of p.223. That's more complex and I don't know that it will work the way you want or need it to for that map. Modern style wooden decorative masks - wall hangings - home decor Umasqu. Or, you can try to create a mask for your cave by creating a multipoly. You can find a link on my video tutorials topic. If you want to know a little more about the Trace command, I did a separate video tutorial specifically about using it. Use the TRACE command to follow the cave exactly, as shown in the video above. Note that you will probably need to use the BREAK command at the doorways/entrances after you create the walls, but that's fairly simple and the online help file should provide clear instructions for using BREAK if you need them.
See Monsen's recent post for a quick description on how to quickly and easily turn a floor into a wall.
The caves on your map were made by applying effects to the cave floor, giving it the illusion of walls by applying a black inner fade along the edges.
The technique shown in the video tutorial won't work for your map because your caves do not have any walls.