These dudes are based on just standard bits of 20x20 MDF wood

I either use a mix of what IJW suggests (using putty) or for packed infantry like these I use a mix of Tamiya textured diorama/soil effect paint. I believe there are equivalents from other manufacturers.

Process is:
- use a lollipop stick to put a thin wedge of the textured paint onto the base.
- Dab of superglue on each miniature and stick on the base. You might have to wiggle it as you do so it kind of buries into the textured paint a bit.
- Once the minis are on the base, use a toothpick or other fine implement to cover the miniature base edges - don't worry about it being perfect. If the paint gets a little flaky add some water to the tool to help smooth it.
- Once is has dried you can add some classic PVA + sand/flock to cover up any imperfections.
This is probably the least painful way I have found of it doing it. And you'll get dozens and dozens of bases from one pot of the textured paint (although make sure you don't keep it exposed as it dries when exposed to air very quickly!)