I also think you should apply heraldry over a fully-painted base, whether it's a shield or a surcoat or other fabric item. If you want to sketch out the basic shape you can use a pencil but I think a coloured pencil works better so you don't have the potential for shiny graphite lines. If you prefer to do this with paint anything soluble like oils or enamels is better than acrylics as mistakes can be smoothed out or wiped off with a solvent-dampened brush (as long as your basecoats are fully cured of course).
One way of approaching a subject like a lion rampant is to break it down into basic shapes as you might see in introductory drawing or cartooning books. You draw rough ovals for the head, the chest and the paws and then join them together. Mike Blank mentioned a technique like this in an article in Mil Mod a while ago.
When you get around to painting the heraldic motif in full you need to do a bit of research in the style of a given era - most heraldry seen in models is completely fanciful and bears no resemblance to what was likely uses on a working shield for example. Bearing in mind these were essentially disposable only the wealthiest would have spent much money on their decoration so flat colours with no shading were likely to have been the norm until they essentially became parade items only; the very rich would probably have been exceptions to this as conspicuous consumption is nothing new.
With something like a surcoat much the same sort of rule applies, heraldic motifs would commonly have been appliqué work - simple cutouts of fabric stitched in place - you can see this in quite a bit of reenactors' gear, and should be painted accordingly. I have to admit this looks quite ugly to our eyes because we're accustomed to seeing something a lot fancier but that's life. For the wealthy (remembering that noble did not automatically = rich) they might have had embroidered decoration, particularly at later dates, so this could be painted to represent the type of modelling and colour variation this might entail, i.e. as we see most heraldry done, from the Continent in particular.
Now colour. Remember the tinctures of heraldry were intentionally very simple to aid recognition from a distance and if you can you should strive to recreate historically-plausible colours. They didn't have Cadmium Red Light or Phthalo Blue or Cadmium Yellow so don't use them for painting heraldry unless you tone them down first! Also remember that even today it's difficult to match paint colours with dyed fabric and embroidery so imagine the trouble they had back then... therefore don't paint everything to match; this is another common mistake seen in the hobby's depiction of heraldry. As an illustration of this there were only three blue pigments that were likely to have been used for most of the era of heraldry's practical usage, of these only one was a nice vivid blue, real Ultramarine. Since this was made from lapis lazuli imported from Afghanistan and extracted by a tedious and drawn-out process it eventually cost more than gold so you can imagine the echelon of society who could have been able to pay for it on their shield.
Last thing, surface. Painted surfaces were very unlikely to have been matt even had they been painted with a matt paint, it's my belief that shields would have commonly been varnished to protect them from the elements. So in addition to the shield not matching the colour of the surcoat it should have some slight sheen to contrast with the matt finish on fabrics.
Einion
:lol: 
