First of all, don't expect to finish painting the white layer on a model in an hour - painting white sadly takes a really long time. I start with Chaos Black undercoat (strange I know but I don't have white undercoat) then use a big brush to go over all the necessary areas in Ceramite White which, as it's a base, has the best coverage. I then paint everything the correct colour so I get an idea of how it will look when it's finished (thanks for the tip James Wappel).
The next step is applying the Drakenhof Nightshade to all the white areas of the model. Different washes really change the feel of the white and I chose blue as a contrast to the green of the Wraithguard models.
When the wash has dried, I paint several layers of thinned-down White Scar to get the white finish. Once I have a white finish that I'm happy with, I shade the darker areas of the model with a mix of White Scar and the Drakenhof Nightshade adding more wash for the darker areas. Any other shading can be done with the Drakenhof Nightshade and a thin brush.
The final few steps are highlighting and shading the other colours. I'll leave the detail of how I do this for other posts.
Do you have any tips or ideas (or better ways to paint white)? Let us know as we're always looking to increase our painting knowledge.