There's a lot of great online tutorials on how to craft terrain for Dungeons & Dragons and other tabletop roleplaying games. Everyone seems to like when I post crafting stuff though, so here we go with my routine.
All my molds were purchased from Hirst Arts. The three molds I use the most often are Cracked Floor Tiles, 1" Trench Plank, and Dirt and Rock. I use these for worked stone, wooden, and natural floors.
Here's my casting setup: just an old baking sheet on top of some foam. The baking sheet is large enough to do four molds at once and the rim prevents (much) plaster from running off. As you can probably tell, I don't mind this table getting messed up. Take precautions if you do, this stuff has a tendency to spill, flake, and get EVERYWHERE.
Safety first! I can't say for sure it's related, but I got a persistent cough one time after doing a couple dozen batches of plaster over a few days without using a mask. I can see the dust in the air while mixing, so it seems worth the negligible hassle to wear a mask or even full respirator.
You're going to want a bunch of clear disposable cups for mixing. I have one that I turned into a measuring cup. First I put a second cup inside it and fill it with water up to the level I think I'll need. Then I take that cup out, measure out the plaster into this cup, and gradually mix it into the disposable "wet" one.
I don't use standard plaster- it breaks pretty easily. I've been using the Garreco Type III Dental Lab Stone. I don't know what any of that means, but I've noticed their Type II is a little softer. It comes in a few different colors and can be purchased on Amazon for about a little over a dollar a pound. (Insert joke here about me preferring glabrezu demons over hezrou) I mix it at a 2:1 plaster:water ratio. For four molds, that's usually half a cup of water and a cup of plaster.
After preparing my mixture and pouring it into the molds, I break out my second-hand back massager. I didn't build a fancy vibrating table, I just push this down and turn the entire craft area into a vibrating table. This gets some of the little bubbles out from the plaster and gives a much better final product. If you can't find a good cheap massager, just doing a drum roll on the table with your fists is better than nothing. After popping the bubbles, I scrape the excess plaster off the top of the molds and let them sit for half an hour. At this point they've set long enough to remove from the mold and start another batch, but I usually wait a day or two to start gluing them down.
I tried a couple batches using double-corrugated cardboard as backing for my tiles, but I found that it warped even more than the foam board does, so I'm back to reliable Dollar Tree foam. I use an x-acto knife to cut out the shapes I need- in this case two 4x4 squares and one 6x6.
I glue my plaster tiles down using tacky glue, but I think any form of PVA works. Even these small rooms would take 2-3 castings, so get ready to get your money's worth out of those molds. The rough edges on the cavern were made by holding a plaster tile across the edge of the table and just pressing down until it snapped. I made it a bit larger because a 4x4 piece with those sorts of rounded edges doesn't give you a lot of room to work with on the game table.
Here they are after flipping them over and trimming the foam from the sides of the cavern and also from a few of the edges. Sometimes the foam square ends up being just a bit larger than the tiles on top, so I trim to make sure they fit flush with other pieces.
The hardest work is over at this point! Really the biggest headache is the casting itself. Most of the time is spent waiting, which isn't a problem, but making a dungeon that requires maybe 200 tiles would be 20 castings if you only have one mold for the tile you want. Obviously once you have some common pieces made, the amount of prep you need to do for future sessions goes down drastically.
I spray paint the undersides black and wait an hour or two.
Then the tops. And wait.
And then do a dark gray for the stone, and brown for the wood.
Time to get out the cheap craft paint! I do a heavy drybrush with a medium brown/gray, followed by a lighter drybrush with a much lighter tone. (Yes, I accidentally bought gray chalkboard paint instead of regular).
After the drybrushing, I like to do a wash by heavily watering down a mix of black and brown and slathering it all over the tiles. This adds a little brown to the gray stone and helps darken all of the cracks. If you go really heavy, I recommend dabbing a bit with a paper towel so it doesn't pool in the flat areas. For the cavern, I throw some washes with a few other colors to make the stone look a little more like it's part of an ecosystem. Usually some yellows, oranges, and greens.
One more trip back outside for some clear matte sealer and we are done! I've mentioned previously some of the reasons why I don't add walls onto my terrain any more, but I'm also glad how much easier this process is without them.
Hopefully that is helpful! Let me know if you have any suggestions or questions.