If your hands are shaky, don't paint. You need steady hands to paint accurately. Don't grip the miniature or your brush hard, this will cause your brush and/or miniature to shake. Use a firm but gentle grip, like holding an eggshell. Brace your forearms on the table edge or against each other to keep them from shaking or wobbling while you paint.

This is one of the few areas that advice won't help. The ability to control your brush comes from practice and paying attention to what you're doing. But without control, you won't be able to paint fine detail.