Sculpting your own chain mail for tabards, shirts, and cloaks is honestly one of the easier things to try when getting your toes wet working with green stuff. The two key parts to it are the proper tools and the right technique. Once you've got those two things, you're ready to outfit an army in custom chain mail.
And yes, I'm even venturing into the realm of video tutorials here with this one.
Let's go over the tools you'll need for this project.
Sharp Hobby Knife (a new blade works best)
Rolling Pin-Like Device
Your Lubrication of Choice
First, I'm going to show you how to do this at a greater scale so that the technique is clear, using some Sculpy, and a larger-sized burnishing tool.
See? Not so hard. Especially considering that all you're doing is making a series of impressions with the burnishing tool, then turning the subject around and making the same series of impressions in the other direction. The only difficult part is making sure you stay consistent with your lines and motions to keep the "chain" consistent.
"Tim, it's all well and good to just do lines of chain on a large, flat surface... How do we apply this technique to the miniatures themselves?". I'm glad you asked. Here, I've got a Guardsman, who originated from a feral world, and we're going to replace the cloth under his flak armor with chain mail.
Two key elements here... Thin and consistent application of the Green Stuff with your modeling tool, and then the appropriate-sized burnishing tool, both well lubricated. After those two, it's the same as the first example.
If you've never worked with Green Stuff before, I hope this has given you a sense of how easy it can be if you just learn and try. I never would have thought that sculpting something as intricate as chain mail on a 28mm figure could honestly break down into this simple of a process... But I remember what Joe Orteza said in a class I took by him.
All you're doing is pushing around Green Stuff.