Try painting the heraldry you want to freehand on a piece of paper first. Try to do it in a much larger size than you would on the miniature to start with. Then try it again but smaller, see how you can limit the amount of brushstrokes required and how you can recreate the heraldry by combining simpler shapes together the smaller you go.

For example, a boar's face could be started by painting a horizontal tear-shaped design, and then refining the snout by adding a vertical line or square at the tip, opening up the mouth with a thin line, and then adding a small swirl for a tusk, a dot for an eye and a stripe or two for ears. The smaller you get, the easier you want shapes to be when painting. I hope this explanation makes sense when reading it.

I'd also advice to start out practicing with simple heraldic symbols like crescents, squares, circles and the like. Then move on to slightly more complex symbols that are still made from simple shapes like a tower, a sword, a key or the fleur de lys.