The boonie hat first entered U.S. military service during the Vietnam War. Its wide, soft brim and ventilated, comfortable crown made boonie-style hats popular with outdoorsmen of every stripe. At its simplest, the boonie hat is a soft cloth crown surrounded by a brim made of slightly heavier, reinforced cloth. A boonie hat also includes an adjustable cloth chin strap that's useful for keeping it snug on your head in windy conditions. When you don't need it to keep the hat on your head, however, the chin strap dangling in front of your face can be a real nuisance.
Put the boonie hat on your head, in the position where you normally wear it. Let the hat's strap hang down in front of your face.
Grab the adjustment slider with one hand. Pull the slider all the way out until the strap's loop is at its maximum size.
Pull the strap over the front brim of the boonie hat and back over your head. Make sure the sides of the strap loop slide up and along the outer edge of the brim on both sides. Use your free hand to guide the strap, if necessary.
Stop pulling when the middle of the strap reaches the center of the hat's crown, if you want to tie up the side brims of the hat. Hold the adjustment slider in place in the center of the crown with one hand. With your other hand, grab the middle of the strap and pull it through the slider until the side brims of the hat fit snug against the sides of the crown.
Pull the center of the strap behind the crown, if you want to get the strap out of the way without altering the shape of the brim. Hold the adjustment slider at the base of the crown in back, then pull the strap through the slider just enough to eliminate any slack in the loop.