Conveniently, I have a Sicaran that needs painting up, so I'll be using this in the example.
To start with, you need your metallic paints - I used to go off the 3 shade rule, using a dark, mid and light version of the colour to give shade and highlights - which completely works and I still do for infantry models - but for the bigger stuff I have added a mid-light shade for a bit more detail.
The colours I use (in lighting order) are Vallejo Gun Metal Grey, Vallejo Oily Steel, Citadel Ironbreaker (the optional one) and Citadel Runefang Steel. You can substitute them out for different brands - the idea is that they are just different shades.
All drybrushing on this example was using Citadels terrain (or big-ass) drybrush. It seems waaaay to big at first but gives a more consistent result than drybrushing big flat areas with a smaller 'large' drybrush. Infantry/samller mini's obviously requires a smaller drybrush.
Finally its the lightest metallic, adding only a little with a fine detail brush to mark the surface scratches.
Now onto the mud; I use Dark Earth weathering powders by Secret Weapon Miniatures, mixing with some matt acrylic medium to form a cross between a paint and a paste (you are going to be painting it on loosely, but its good to have a few clumps of thicker powder/paint for texture).
Using a pretty big old knackered brush, I stiple the mix onto the tracks and anywhere I think mud is going to fly off onto the tank.
Thanks for dropping by folks!.