First abandon the base 10 numbering system, and move to base 2 (binary). Then hold both hands out with your palms facing you. Now your right thumb is 1, right index is 2, right middle is 4, right ring is 8, right pinky is 16, left pinky is 32, left ring is 64, left middle is 128, left index is 256, left thumb is 512.

Examples:
19 = right thumb + right index + right pinky -> 11001

40 = right ring + left pinky -> 000101

378 = right index + right ring + right pinky + left pinky + left ring + left index -> 010111101