Painting a flag instead using a printed one can give more challenge to the painting , and n my opinion it melts better with the miniatures itself.

1-Get the right informations and sources: internet is more and more the place to find it ex NAPFLAGS , or the very detailedROFUR FLAGS ; or better by the 3 Osprey books  .

2- Build the flag : I make the flagpole with metal wire and the flag itself with paper, being careful to size the exact scale dimension, the paper is folded on the flagpole and fixed with glue. I add the flag edge if existing out of thick paper and the cravats out of papers and cords made with electric copper wire.

3- Primer and preparation: I spray primer (grey) on the miniature and i cover the flag with white color as base for the drawing.

4-Design of the flag: I design then the pattern of the flag, with pencil, if you keep the flag unfolded is easier to do the work, exactly as designing on a piece of paper.

5-Base colors: i leave the white untouched and paint the main basic colors, for the gold i paint a base of ochre  first.

6- Relief and contours: i carefully highlight the contours with black glaze and if necessary with very diluted black.

7-Highlights and  gold: I then highlight the main patterns with lighter nuances, and i paint over the ochre with mix of gold and ochre itself. 

8- Final contours and details: i finalize the contours with the same mix of point 6; and i paint the lance, cravats and cords, stave and the fixing bolts of the flag.

And don't be afraid to make mistakes, you can always correct them in the process.