There are lots of great decals available for 28mm dark ages figures, and fair play to you if you use them. For me though, much of the pleasure I get from the hobby is painting my figures, and I enjoy the challenge of trying to paint tricky shield designs. Last time I put some images of Welsh figures up on the blog Sean suggested I do a tutorial explaining how I go about it, so I hope this might be useful to someone.
First up, you need inspiration and ideas. There are lots of good cheap books available on Norse and Celtic designs that will give you ideas. Probably the classic is 'Celtic Art: The Methods of Construction' by George Bain. If you are feeling cheap, a quick google image search for things like 'Celtic Spirals' or 'Celtic Crosses' will turn up lots of designs.
When I find a design I like, I often have to think about how I can make it work on a circular shield with a boss in the middle of it. It is useful to do a rough sketch at this point. For this tutorial I have drawn some sketches showing the steps I take when painting different designs, but usually I just do most of this in my head.
First up, here is a knotwork pattern. I use paint thinned with a little water and a size 0 or 1 brush to put dots on the shield to help me construct the shape. Then it is just a matter of following the design steps to draw up the design, before using the background colour and a size 0 brush to create the over/under interlacing effect.
Below - completed apart from the interlacing.
The drawing below shows how to create a simple but effective triangular knot that looks great on the arms of crosses, for example.
The figure on the right has the triangular knots without the final interlacing, while the interlacing on the left figure is complete.
Here is a drawing for a more complex circular design. It is actually easier than it looks - the key is to set up some faint construction lines before you begin.
Spirals are probably the type of design I find the hardest. Below is a drawing of a spiral terminating in animal heads, inspired by a design from Bain's book. I struggled a little with the proportions of this, and had to abandon my first attempt.With beasts generally the final outline is added with slightly thinned black or white paint and a 0 or 00 brush.
When the figure is finished, I add shade and varnish using the acrylic alternative to dip I discovered. After matt varnishing the figure I add some highlights to the shields. Oh, and generally I tone down colours, adding a little buff to colours like red and green, or a touch of grey to white.
Hope you find this useful, or at least feel inspired to continue using the decals!