In the "create tournament" dialog, there is a line "entry fee" which allows to set the amount in given limits. Be sure to set the "Credit Payout Rules" to your liking - it may be advisable to choose "Top 10" as a standard and to be reluctant concerning cut lines.
The following procedure is widely identical to public premium tournaments. Players pay their fee, WGT takes the rake, winners are payed following the rules in the FAQs.
One caveat: If there is no winner, WGT takes it all. This may happen when nobody posts a score, most likely in multiround tourneys and possibly facilitated by cut lines.
Have fun!