Using the brush, apply glue to part of the outside edges of the mirror frame: set a straw in place, add another, cutting off the bit of straw that juts out and so on... Apply glue to the rest of the outside edges, stick on straws, then do the same to the edges of the mirror itself.
The area where the straws will be stuck has now been defined. In this area, you have to form big rectangles or squares, blending different combinations of colors in each area, it'll be even prettier like that! Apply glue to a smallish area, then position your straws on the frame.
Continue like this, sticking on more straws bit by bit. Remember to place the straws vertically, horizontally and diagonally too (which gives me the opportunity to learn a new word that I really like!).