Personally I build the entire model, do the basing, and then prime the entire thing by brush using Black . Order is not super important, but there is some time to be saved by washing both the fatigues and the flak at the same time.

Fatigues

1. Basecoat Zandri Dust
2. Wash with Agrax Earthshade / Lahmian Medium 50/50 mix
3. Reapply Zandri Dust avoiding recesses
4. Highlight with Ushabti Bone
5. Highlight with Screaming Skull (optional)

Flak armor

1. Basecoat Vulkan Green
2. Wash with Agrax Earthshade / Lahmian Medium 50/50 mix
3. Reapply Vulkan Green , avoiding recesses
4. Chunky highlight Loren Forest
5. Fine highlight Krieg Khaki

Flak camo pattern

1. Paint triangular shapes with Skrag Brown
2. Paint shapes' edges with Karak Stone
3. Fine highlight with mix of Karak Stone and Skrag Brown

Weapon casing

1. Basecoat Nocturne Green
2. Chunky highlight Death Korps Drab
3. Fine highlight Nurgling Green

Eye lenses

1. Basecoat Thousand Sons Blue , leave top still black
2. Chunky highlight Temple Guard Blue
3. Fine highlight Baharroth Blue
4. Dot corners with White Scar

Black leather

1. Basecoat Abaddon Black
2. Chunky highlight Eshin Grey
3. Fine highlight Mechanicus Standard Grey

Black casing (scopes etc.)

1. Basecoat Abaddon Black
2. Chunky highlight Dark Reaper
3. Fine highlight Dawnstone

Metallics

1. Basecoat Leadbelcher
2. Wash Nuln Oil
3. Highlight Ironbreaker

Recommended order

1. Paint faces or lenses
2. Base fatigues
3. Base flak
4. Base black
5. Paint camo pattern
6. Wash with Agrax Earthshade / Lahmian Medium mix
7. Base metallics
8. Wash metallics Nuln Oil
9. Highlight metallics
10. Highlight flak/weapon
11. Highlight fatigues
12. Highlight black/leather
13. Paint small details (lamps, scopes)

Bonus: Basing

1. Apply small rocks, sand + PVA
2. Prime black
3. Paint Dryad Bark , add spots of Mournfang Brown while still wet
4. Drybrush Karak Stone
5. Drybrush light Screaming Skull
6. Rocks: Dawnstone Nuln Oil Administratum Grey Corax White
7. Add static grass of your choice
8. Rim: Corvus Black