Via the commission widgets on their page, or the "give" button after letting the artist know what you want. (If they're willing to do it) You can pay with points which can be earned by opening commissions, something about someone named dAhub I believe? I forgot how to earn them from there. Or by just friends and other nice people giving you free points. Or, if the artist accepts real money they'll say it in a journal (most likely) and it's usually through paypal if it's down money lane.