Caz, login to your Sedo account, go to My Domains, tick the boxes next to which names you want to auction, then at the top of your domain list there's a tab called Promote Domains, click that and a menu opens, select GreatDomains Auction and then follow the instructions.
Note that you can only add 5 domains in any 24 hour period. Be careful with your reserve prices you set, if you set too high you'll get an email saying it has not been allowed to enter auction as reserve to high, if that happens, that bars you from ever entering them in any future auction via this route.
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Any names entered now are for the next auction on Mar 21st, there's 1 GreatDomains auction per month, or is it 2, hmmm, I think it's 1