A neatly folded handkerchief, or pocket square, is a nice way to finish off the look of a suit or sport coat. The only rule of thumb about the color is it should match one of the colors of the tie. There are several ways to fold a handkerchief for the suit pocket, with the three most common being the straight fold, the puff fold and the one corner fold.
The Straight Fold
Lay the cloth flat on a table.
Bring the left side to the right side, then bring the bottom toward the top but not all the way.
Fold the handkerchief in thirds horizontally until it is small enough to fit in the pocket. A small rectangle should peek out of the pocket.
The Puff Fold
Lay the handkerchief flat.
Pinch the middle with one hand and use the other hand to gather up the bottom.
Place the handkerchief in your pocket. Adjust it so just a "puff" of the handkerchief shows.
The One Corner Fold
Lay the handkerchief down so it is in a diamond shape.
Bring the bottom point to the top to create a triangle.
Bring the right corner to the left corner. Bring the left corner to the right corner. It should look like a rectangle with a pointed top.
Place the handkerchief in your suit pocket. Adjust it so it shows just the amount of fabric you want.