First, you must create the profile you want.
This can be done inside The document Profile Folder
(Wrkspace/Management/Document Profiles).
Double click on the "Add Document Profile". It is a pretty straight
foward process with the wizard, so I don't think you will have
problems figuring it out on your own.
After you do as such, go to the Document folder you wish to set that
Profile with.
Right click it and open its properties. Select the "Profiles" tab.
This is where you say which profiles are available for documents that
are checked in to this folder.
If you check only the new profile you created, that is the only
profile which will be available for the user. If not, he will be able
to select between those you leave marked.
Each profile will bring its appropriate fields.