I took some pictures while making the bases, to make a quick tutorial on how I do my Nurgle bases:
I. Roll a lot of small balls in some kind of sculpting putty. I use green-stuff or milliput, depending on which putty i'm working on at the moment. Usually i just roll a couple of eye-balls with the left overs when i'm done sculpting.
I then make a base and randomly glue them on. be careful not to glue them where the model is supposed to stand.
II. Paint the base like the rest of your army. This base was painted like the World Eaters bases, but with some green and sepia glazes to give them a more Nurgle look.
III. Start painting the eyes white.
Then pick out irises with different colours. I randomly pick out the irises with red, green and blue. It's important that the iris is the correct size, if they're too big, it won't look like eyes anymore, and if they're too small, they will be covered by the pupils.
paint the pupil by adding a small black circle in the middle of the iris.
IV. GW's "blood for the blood god" is painted on the bottom half of the eyes. Try to make this look natural by not having a straight line around the eye. Stipple some blood on the ground around the eyes to blend them with the base.
Finish of the eyes with a layer of water effect (or any other transparent gloss paint).