If I told you how I do it, I'd have to kill you...
I AM NO EXPERT BY ANY MEANS! There are a lot better than me but I consider myself a guy with a little better than average skills.
My way is relatively easy. 95% airbrushing. Primer, paint, I then line every edge, corner, and places where there would be outlines with .25mm drafters marker/pen. I then do some shading by taking the original colour and darkening it a lot and cut it 5% to 10% paint to thinner and air brush very lightly along seams, corners, places where shadows would be. For the flat open areas do the reverse. Lighten the original colour and do the same thing. Do both lightly and let it dry. Look at it and do it again if it needs more.
Once you are happy with the shading then spray clear acrylic floor polish over all the model. It is at this stage you apply decals and do either a black or dark brown pin wash. I just use dollar store paints for these browns ivory blacks etc. and cut them down at least 80% water. This blends into the the black lines you did at the beginning. The floor polish allows the pin wash to have capillary action and run around things like hatches, bolt heads, weld lines, etc.. This is also when you can use an HB pencil to do edges to give metal appearance.
For chipping tedious, time consuming put on some Pink Floyd pour a tall cold one to keep your sanity On the Comet I used buff and a 000 brush and do the edges of hatches, edges of grill, scratches etc.. and touch, touch, little stroke, touch, touch, a little here, a little there, and before you know it 4 or 5 hours have passed. Now you go back and in the Comets case I used an almost black green and do the centers of the buff chips you just painted and kiss another 4 or 5 hours goodbye! LOL!
For streaking and this one kills me! There are companies out there that sell these rust washes, engine grime, streaking formulas, etc.. and all they really are are oil paints thinned down which you can buy for a fraction of the price! Go to a artists store and find the cheapest oil paints you can find. Things like burnt umber, raw umber, and other colours that interest you. Take them home thin them down to a little thicker than water consistency with colour in them. Use a small brush and also from the artist store buy a large round 1" bristle brush. Practice on some painted styrene by putting a couple of little dots on an edge leave it for 20 or 30 seconds and then take your bristle brush and pull it straight down and viola! Streaks! Here is an example on a ZTZ99A

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Now for dusting! It is at this stage I switch from Tamiya acrylics to clear flat lacquer.The reason I do this is I build tanks that fight and take some abuse so the Lacquer is much tougher than clear acrylics but you can use regular acrylic clears of course. If you have trouble finding it look at railway model shops they usually have it. Cut it 50/50 with thinner and add a TOUCH of things like dark earth, mud, whatever colour you want for making the lower hull dirty. Again light thin coats till you get it the way you like it. Start at the bottom and then less as you go higher. Last I will open my airbrush up and spray from about 2 feet away in bursts all over the tank. Next mic the clear lacquer with something lighter like buff or sand and do the higher areas. and you can again do the open flat areas with this mix too. Do any touch ups with a pencil pin wash etc.. and it should be good to go...
If I forgot anything sorry! Of course doing camo tanks is a little different...

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