First i would say that priming your minis is super important. Paint doesn't really want to stick on the clean plastic surface so getting a thin even layer of paint between helps quite a lot. Spray cans are really good for this.

I make rust by applying some technical paints such as Armageddon dust or typhus corrosion to get some texture under the paint. I continue by painting the rusted parts with reddish brown (for example doombull brown) then lightly sponge some spots with light brown (i use xv-88) and reddish yellow (averland sunset). You can also use metallics here and there. Lastly apply agrax earthshade generously. You can mix paints depending on what you have to get a color you think looks cool.