Ummm. I just paint the design on, in a logical way...
But... for example if you were doing say a Starship Troopers MI standard bearer...
Paint the background colour over the whole flag; in this case green...
Paint the white circle and the diagonals in white.
Over the white leaving a border paint the blue interior.
Paint the inner 'Eagle' and diagonals in white.
If you told me what the design was I could break down the pattern/design into the logical painting order for you if you so wished... Or just break it down yourself in a logical way...
Making it all sharp and crisp as a design, well that's just practice.