Dear LOers, welcome to the first in (hopefully) a series of informative guides I am producing to preserve my knowledge throughout the generations.
Now, I'm not going to start at the beginning. I am planning a guide on how to use various tools, how to pin, how to clean mould lines etc (ie: the bare-bones-basics), but this is not it.
This is an article about how to make consistant 3d green stuff chapter bades, army symbols etc.
I had a lot of people ask me how I made the ones on my Black Dragon minis (see various posts in the Painting and Convertion forums), and I have finally got around to it. With pictures! I am using my Black Dragon icon as an example, because it was what I had in front of me at the time.
Anyway, on with the show!
In this guide, I am assuming you have used GS at least a bit before, and I will not cover how to work with it. There are some great guides out there, have a look on Google, or PM me if you need some help =)
The Tools: 
Here are the tools I used for this technique.
- Scalpel - For cutting, and for fine GS sculpting.
- GW sculpting tool - For basic GS shaping.
- Rubber "smoothing tool" - For GS smoothing and shaping.
- Needle Files - For filing.
- Green Stuff - I'm sure you can figure this one out...
- Vaseline - Lubricant.
- You will also need an old brush, pencil, or piece of dowel or plastic rod.
I'm not claiming that you can only use these tools, or these are the best, but these are what I like to use.
I know a lot of people don't like GWs sculpting tool, in which case use whatever you prefer.
I use vaseline as my GS lubricant as it is more effective and easier to control than water (in my opinion), but if you don't have any, then use water (or whatever you prefer).
The Process: 
First, mix up a small blob of GS, and apply it to a spare shoulder pad (only if you're making badges for your space marines... If you want icons to go on flat surfaces, do it on some plasticard or something flat, if you want icons for something else, do it on a spare bit of whatever that it...).
Get it smooth and flat to the surface, about 1mm or so high.
Wait for it to dry a bit, then create the icon you want using the GS. This is the hardest bit of the entire process, so if it isn't going well don't worry. Keep trying, and don't worry if it isn't perfect first time. Just keep going until you do get one you are happy with. This is what we will use to cast the mould.
Wait for a day or so until it is cured (to cure completely takes about a week or so, but after a day it will be fine).
The next day, mix up quite a big blob of GS, and attach it to the end of your old brush (or pencil or dowel or whatever). Apply lots (I mean lots) of vaseline (or other lubricant) to the GS icon you carved yesterday. Try to get it so that it covers it evenly, with no lumps.
Push the big lump of GS onto the icon. Try to get some of the edge of the shoulder pad (if that is what you are doing) so you can always align the icon properly.
Pull your new mould away from the icon, and see if it is properly formed. If it hasn't worked first time (mine didn't), keep trying. Leaving the blob on the icon for 15 mins or so may help.
Leave your new mould to dry for at least 2 nights.
When it is finished, you should have something like this.
Good huh? This is your mould. Guard it with your life.
When you want to make a new icon, mix up a small blob of GS, and smooth it over a new pad, like you did earlier. Then, put plenty of lubricant in the mould, and push the mould into the GS
hard. Leave it for a few minutes, then remove the mould.
You should have something like this. If not, smooth it out, and try again.
Wait for it to dry, the cut away the excess GS.
Congratulations! You have now cast your first icon/badge/whatever. You can now put the pad onto your finished mini, and paint as ususal. A painted and an unpainted example of mine are shown below. Both were made using this method.
I hope this guide has been useful for you, please feel free to PM me if you have any questions =)
-t.