Regina - you just manually upload using the 'Add new' radio button in the top right-hand corner of RG. The process is relatively straight-forward in terms of adding required publication detail. If you wish to upload your article (for private or public access), and you don't have an existing publisher pdf copy, then you will have to manually scan and upload that file.
Be sure to check the copyright rules of each journal. Some may not allow you to share any version, some may allow you to share their printed version, or some may allow only sharing a "pre-print" version. "Pre-print" is what I usually share (if the journal allows it). If you are not allowed to share any version, you can still share the abstract when you create the publication page in RG and provide the link to the journal's page for your paper.