Just because underwear is worn on the inside, it doesn't mean that piece of clothing doesn't need to be ironed. After all, you never know who might see it. So before you take your underwear fresh from the dryer and stuff it into your dresser, take a minute to warm up the iron.
Determine the kind of fabric you will be ironing, like cotton, silk, rayon or polyester. All of these fabrics respond to different temperatures. Check the tag for material and washing instructions. Very often it will suggest whether or not the garment should be ironed.
Turn on your iron to the setting that corresponds with the ironing instructions with the tag. If the tag doesn't give instructions, set the iron by fabric type. If that isn't present on the dial, start the iron off on low. You can always crank up the dial if the heat is to low as it's best not to scorch the fabric.
Iron the fabric along with the pleats and natural folds in the garment. Obviously this is different for a bra, piece of lingerie or a pair of male boxers. Common sense is your greater guide here.
Fold your underwear carefully after you are finished. You've just done the hard work, don't simply toss the underwear aside again; the clothing will become wrinkled.