dunno about realistic (check out my freebies, the palest ones) - but the way i do it is set the brush size to fall off in however many number of steps, then... just brush around... :) choosing the colour and opacity is also important. sorry to be so useless there, i can't explain without pictures.

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a sweet disorder in the dress kindles in clothes a wantoness,
do more bewitch me than when art is too precise in every part