You can, of course use shield transfers. I have played around with them but don't really like them.
I think a painted shield looks much more realistic. Of course it will not be as perfect as a transfer but the actual men would have hand painted their shields themselves without the help of 21st century computer generated images. I have occasionally used transfers for particularly complicated medieval heraldry but then I paint over it to give it a more realistic look that blends better with the rest of the painting.
With a complex shield pattern I find it easier to paint the shield separate from the figure and glue it into place after painting, accepting that the bond will be less durable than a bare metal shield glued to a bare metal body.
These figures will have fairly simple shield patterns. I have outlined the base design with black ink, using a very thin high quality brush. This divides up the sections which will be coloured in after and make them stand out. I do not always do this.
You can see the central figure where I painted over with a bit of white to correct some earlier mistakes.
I then paint the rims and add in any final details. Most of the rims are Coat d'Arms Leather Brown. Some are red leather created by adding a bit of red to the base leather colour.
When this is done I apply a very thin wash of Liquitex Transparent Raw Umber over the entire figure. This brings out the detail and draw the colours together.
I apply the wash before painting the metal areas as the wash will dull the metalic effect.
Next, I paint in the basic colours, not worrying about the paint covering up the rims. When the base colours are done I paint over the rims with white.
Most shields were rimmed with leather but there is evidence of some metal rims. For a metal rim I would use a black rather than white.
The figures are now finished. You can see how the raw umber wash has brought them to life. The hair and caps look very realistic and the detail is picked out. All of this has been achieved without highlighting, outlining or dry brushing.
After the Raw Umber wash I paint the metalwork. I use Games Workshop's Boltgun Metal highlighted with Silver. For Bronze I use Shinning Gold. To paint the metal studs I first used a dab of black then the metal on top. This gives a partial outline that helps the studs stand out.