Start by using red face paint to sketch out the lines on the Union Jack. Next, begin filling in the spaces with royal blue. Make sure to leave an edge for the white lines.  

Continue to fill these in, in the shape of the mask. Then use a light blue to add highlight onto where the light would catch the mask. 

Before adding the white ensure all your paint has dried, this is important to make sure that the colours don’t mix and make the white murky. To finish, add black around the edges to create a shadow and give the mask the appearance of being 3D.