Once you have completed the GEDCompare (the process by which your GEDCOM is compared to existing WikiTree profiles and marked each potential match as accepted or rejected you will get ADD buttons next to each person in your GEDCOM. You will then use the ADD buttons to create each profile one by one. The data section of the profile will be pre-filled. The biography section will have to be "cleaned" of what is referred to as "GEDCOM junk" and your sources may very well need to be reformatted.
There are several pages to help you with this process. If you will go to the Help Index and do a search for "GEDCOM" and "GEDCompare" you will find most of them.