Go to Sedo domain manager and select the domain(s) that you want to submit for auction. At the bottom of the page you will see a box with "Save marked changes" and an execute button next to it.
Click on the small blue down arrow next to the SMC box, this will open up a list of options. Select the auction you are applying for and click the execute button.
Another page will appear with the selected domain(s) at the bottom of the page, here you can set your reserve and then send your application.
It will be around 24-48hrs before you get notification of acceptance or rejection.