Barren tan terrain:
For my Star Wars miniatures I decided to make plain tan bases with no green vegetation, so the figures would not look out of place on barren alien terrain.
To make a 1 inch diameter round base I selected a 1" wide washer with a 3/8" diameter hole. These cost a few cents apiece, and are least expensive when bought in boxes of 100. Fender washers may be selected instead -- they are thinner, have smaller holes, and may cost several times more than ordinary washers.
The washers always have some machine oil on them, so first I wash them in soapy water and dry them on paper towels. Some washers may have burrs that keep them from sitting evenly on the table and that might snag a terrain drop cloth. Burrs can be removed by rubbing the washer across a broad, flat file (an ordinary steel file which can be found at a pawn shop for around a dollar). The easiest way to prepare the bottom of the figure for glueing is to rub the figure's base on the file, holding it over a paper bag to catch the shavings. Then I superglue the figure to the washer and let it dry completely. The extra time taken in cleaning the parts results in a much stronger bond.
Most 25mm figures have bases that will span the hole in a washer, though a lot of the base surface will be over the open hole. If a large part of the hole is peeking out from under the figure's base, I cover the gap with a scrap of card or plastic cut from a blister pack. Tiny gaps can be ignored because they will be filled automatically by the later steps.
The textured surface is Woodland Scenics medium model railroad ballast, which is supposed to look like crushed rock in model railroad scales and will look like fine gravel in 25mm scale. A 7 oz bag costs around $2.50 and will last a long time. With one bag I've covered several platoons of figures and used some to texture terrain pieces and the bag still looks full.
Once the figure is securely glued to the washer, I squirt some white glue onto the washer and carefully spread it around the washer top and onto the figure's cast base with a stick, trying to keep it off the sides of the figure's feet and legs. I leave a tiny gap around the feet so they will be fully visible, and this usually creates the illusion that the figure is standing on the textured surface and not sinking into it. I put some model railroad ballast into a small jar and dip the figure's base into it until it is covered, then remove it and gently tap off the excess. I'll check to see if any part of the base needs special attention, like removing some ballast from the top of a boot or filling a little gap where ballast is missing. Hopefully the base will have a smooth contour that hides the bump where the figure's original base is, although the ballast may settle a little bit as the glue dries.
I usually will let the white glue dry overnight, then use an old brush to paint some watered down white glue over the top of the ballast covered base. That extra coat of glue will help keep the ballast attached and make the base more durable. This has to be done carefully or else the watered down glue will soften the first layer of glue and the ballast will start to loosen.
Then I spray prime the entire figure and paint it. I paint the base last, since some paint is sure to run or drip onto the base while painting the figure. I paint the base a tan color that I mix from one part Delta Ceramcoat Brown Velvet and several parts of any white craft paint (I don't use a strict formula -- the mix is right when it looks like sand). I wash it with brown or reddish brown paint or ink, then drybrush it with at least one lighter shade of sand colored paint. I have a habit of finishing by painting an extra layer of tan paint around the side edges of a base, since the edges will take a beating during play and handling.
It can be deadly boring to sit down and glue ballast to a lot of bases, but you can do a platoon's worth of figures in one or two nights in front of the TV and get them all out of the way at once. I have done over 50 figures in one batch, with the result that I could paint for a long time before I had to do it again.