I made a quick step-by-step for the Samaria in Ashes group for these, I'll share that here as well:
1) Put some filler in the bases. I used cheap spackle. This doesn't have to be in any way smooth or even. Wait for it to dry.
2) Find some electrical components. This could be anything, mine are mostly computer boards. Use a plate cutter and cut out pieces of appropriate size. Both single pieces and several smaller pieces will look good. Roughen up edges as needed, and break off any transistors or other parts you don't want.
3) Using strong glue (I used Gorilla Glue), attach the pieces to the bases. Let dry completely.
4) Using superglue, add more pieces to the bases, this could be screws, cogs from watches, wires, mesh, parts of sprues, anything you'd like. Let dry again.
5) Water down some PVA glue, and put some on the bases in all places you don't really like - gaps, edges, anything. Dip the bases in fine sand, and let the sand sit on them for a few minutes before you shake off the excess.
6) Add more parts if you want some to be untouched by the sand.
7) Spray the bases with a clear coat. This is your 'primer'. I used a cheap glossy coat for this.
8) Wash the entire base with Agrax Earthshade or a similar wash. If you want them less dark, water the wash some. Let dry.
9) Add some Nihilakh Oxide. Don't overdo it! Add a little here and there, either small amounts or larger amounts where you wipe away the excess.
10) Add some Tyhus Corrosion. Again, don't overdo it.
11) And we're done! At this point, you can add some weathering pigments if you have them (Which I will).