1) Grab a cheap set of make-up brushes in the closest discount store or through online retailers
2) Prime with Grey Seer or any other light grey primer. Make sure you do this in a well ventilated area!
3) Wait a couple hours for your primer to dry!
4) With the biggest make-up brush you can get, drybrush Ulthuan Grey everywhere
5) With the same brush, drybrush White Scar. Now focus on the upper parts of the terrain and places where the light would hit from above
6) With a smaller flat make-up brush, drybrush (or better said, stipple) Balthazar Gold in the windows, floor sections and Mechanicus skulls. Dont worry if you hit the walls, these mistakes will look awesome as scratches after we wash everything in the next step!
7) Wash everything with Agrax Eartshade. Do one side of the each terrain piece at a time to avoid making a mess.
8) And thats it! You have now speedpainted your terrain!