What colour do you want them to be?
Easy options would be something like:
Soft Green Nurglings:
White Undercoat
Athonian Camoshade Wash
Pallid Which Flesh Drybrush
Blood for the Blood God for pustules and boils
Darker Green Nurglings:
White Undercoat
Straken Green Basecoat
Athonian Camoshade Wash
Nurgling Green Drybrush
Blood for the Blood God for pustules and boils
Pale Cream Nurglings:
White Undercoat
Zandri Dust Basecoat
Seraphim Sepia Wash
Ushabti Bone/Screaming Skull Drybrush
Blood for the Blood God for pustules and boils
Each of these methods means you can use the airbrush or existing primers to base coat the model. Then it's just a wash, which can also be done using the Airbrush, then a dry brush to lighten things further. Any boils and things can be picked out using BFTBG because it's super simple and achieves a good result with only 1 coat.
Again though, give us an idea of what colour scheme you want to achieve. Also, what level of painting standard you want...that will affect the advise you receive.
