I did it for a whole
DG army. First, get this tool, it makes life sooooo much easier when the model is already glued on:
https://www.games-workshop.com/en-US/M-Texture-Speader
Use stirland mud. let dry, then apply like a forest green (dark angels green if your sticking with citadel). When you use the stirland mud it will create little "valleys," for lack of a better word. Paint the dark green in those valleys. When you've already glued your models I usually just do the front and back of 25-32mm models but got a bit crazy on the larger bases.
Then use the Nurgle's rot technical over the green paint:
https://www.games-workshop.com/en-US/Technical-Nurgles-Rot. When it's all done it comes out like the ooze from ninja turtles and very nurgle, and very toxic.