Going to appologize in advance for these being Fantasy Football rather then marine miniatures, but, I've never put braids on my marines ;)
I've done braids in GreenStuff in 2 different ways, depending on the effect I wanted.
The first thing I do is roll out a really thin snake of GreenStuff, which I then chop into 3 or 4 equal lengths (which should be longer then your intended braid. Now, let the braids sit for ~30 minutes so they stiffen a bit.
In the case of a twined braid, I take 3 snakes and pinch them together at one end, then literally braid them by taking the furthest strand left and bringing it to the middle, then the furthest strand right and bringing it to the middle, and continue to alternate furthest left then right until the braid is the length I need. You can then drape it in place (using the top and bottom excess to hold and manuever it, then cut those edges off once it is positioned.
example in the smaller braids on the below miniature.

The other style is the larger center braid above, and on the mini to the right below.
This is a bit easier, but is more of a pleated braid (might look good for dwarf style braided beards).
1 - You take 2 GS snakes and twine them together. Basically, pinch the ends together between thumb and index finger on each end and twist in opposite directions. So, left hand twists towards you, right hand twists away from you. Make the twist as loose or tight as you like.
2 - Take the other 2 GS snakes and twine them together, but do the opposite of what you did last time. So, left hand twists away, right way towards you.
3 - Now take the two twined braids and press then together so the twines mirror each other.
4 - Pinch the ends, place over area you want and cut to shape.

Tips :
Wear smooth latex gloves when rolling and working them - leaves no finger prints :)
You can also drag a thin blade along the snakes/braids to give the hair texture.
I hope that all made sense, maybe when I get home I'll get around to taking photos to illustrate each step.