My experience:
1. Tell HR you want to hire a postdoc.
2. HR posts an ad in the university's job site.
3. Ask people apply through the university job site.
4. Get some applications.
5. Tell HR which one you want to hire and why he is the best fit.
6. HR send out an offer letter.
7. You future postdoc signs the letter and return it.
8. HR send out hiring paper work.
You can advertise the opening by yourself, but in my university the final
hiring has to go through the job site. If you already have