This is based on RobPaintsModels method (may he rest in peace), with minor tweaks for some details.
Please watch his videos to see how he paints his DW, as my scheme is 90% following his and his videos are excellent!
Part 1: https://www.youtube.com/watch?v=VFBfnzxUVs8 (Black Armour and Edge highlighting)
Part 2: https://www.youtube.com/watch?v=Hkr1qKSPAmc (Red and Silver)
Part 3: https://www.youtube.com/watch?v=JaKdxgHbDxE (Leather, Gold and Snow base)

Prime

1. I used MrHobby Mr. Finishing Surfacer 1500 Black, but any black primer will do, such as Chaos Black .

Armour

1. Base: Abaddon Black + Kantor Blue (Both Citadel Air) - I airbrushed this on.

I mixed up a bottle of this as Rob suggested, calling it 'Deathwatch Black'... it can be used to very easily tidy up any mistakes later on.

1. Metal Base: Gunmetal Metallic - Do all metallics, including back knife, symbols (whether they'll end up silver or gold), plus armour joints.
2. Metal wash 1: Nuln Oil
3. Metal highlight: Silver RLM01 Metallic (VMA Silver)
4. Metal wash 2: Nuln Oil + Abaddon Black mix, into the armour joint recesses to darken them.
5. Armour edge highlight 1: Russ Grey - All edges/details (top and bottom).
6. Armour edge highlight 2: Fenrisian Grey
7. Armour edge highlight 3 (spot): Ulthuan Grey

Any mistakes can be fixed using the DW Black mixture.

Armour Scraches

1. Use Fenrisian Grey and apply scratches in thin lines, starting at highlighted edge, or adding crosses.
2. Glaze: Abaddon Black + Kantor Blue + Water (i.e. thinned Deathwatch Black) and glaze over the ends of the scratches to create a faded effect.
3. Highlight (spot): Ulthuan Grey - Add to middle of crosses or where scratches meet the edge highlights.

DW silver arm

1. Base: Silver
2. Wash: Nuln Oil + Abaddon Black
3. Highlight: Chrome Metallic
4. Diluted wash: ?? Diluted Leviadon Blue or Drakenhof Nightshade ?? To be tested to get a distinct 'DW silver' that's different to other silver details.

DW silver arm motif (skull and cross bones)

1. Base: Retributor Armour
2. Wash: Reikland Fleshshade
3. Highlight 1: Liberator Gold
4. Highlight 2: Stormhost Silver

Other silver details

Lenses

Red eyes

Red details

Leather

Purity seal

Weapon metal

Weapon red

Weapon gold

Weapon handles