To tie a bow on a dress, make sure the sash is flat on the dress and both sides are even. Cross one side of the sash over the other, then bring the top side of the sash behind, up and over the other side. Pull both sides to tighten.
On one side of the sash, make a loop. Hold the loop with one hand and use the other hand to bring the second side of the sash across and around the loop. Draw the second side of the sash through the loop you've just created, forming a second loop (as you would with shoe laces). Pull both loops gently until the bow is tight, then smooth the loops and any sash excess.