If they are quality leather khussa:
Wet your feet and wear your khussa around the house for an afternoon. Occasionally rewet your feet. If they pinch or rub anywhere, you can put on a band-aide with a cotton ball under it, to help stretch the leather in that spot (i.e. on the back of the heel or on your big toe). Every time you wear your khussa they will become more comfortable as they mold to your feet.
If your khussa have quality man-made uppers or inserts:
Putting wet feet into synthetic shoes is not a pleasant experience! If your khussa are not leather, put on a pair of thick socks and wear them around the house for an afternoon. Just like with leather khussa if there are any spots that rub, you can put on a band-aide with a cotton ball under it (though this will not stretch the area as much as it does with leather khussa).