To start, I squirt some glue blobs with regular white glue around the base and then wipe it into a layer with a piece of wire. Anything will do, really, but wire is what I like to use since I can kind of bend it into a shape if I need to work around the legs of the mini.
Then I dip the base into a mixture of fine-grain and medium-grain sand I've mixed together in a ziplock bag. I don't do anything special, just dip it and shake off the excess sand.
Then I let it dry for an hour or two. You don't want to try and paint the sand when the glue is still wet, because you'll move it around and make it look weird, and then you'll have to try and mash it back down again which is a pain. Just let it dry.
To recreate the mine floor base color, I make a slightly watered-down 50:50 mixture of Dark Flesh and Leather Brown (all colors from the Shadows of Brimstone paint sets), and paint that onto the glued rocks and sand until everything is an even brown. I water it down a little because that allows it to get in all the cracks between rocks and cover the whole base. Then I let that dry for about an hour or more. Since I water-down the paint it needs a little longer than normal to dry.
I like these bases as I can do them really quick and they look good on the Brimstone tiles. If you feel like putting some extra details or something on some of the larger monster bases you could in the future, but I'm a fan of clean, simple bases so these are good for me. Also since these minis will be handled a lot I recommend doing a final coat of Matte Varnish over them to protect the paintjob.
So there you have it. Quick and dirty bases for Shadows of Brimstone.