When you are viewing an image in Lightroom, and you choose to "Edit In" Photoshop, the round-trip process is automatic. Simply make your Photoshop edits, and then close the file, saving the changes. It will be returned automatically to Lightroom.
One thing to avoid: never "Save As" in Photoshop, giving the file a new name. If you do, Lightroom will lose the link, and the edited image will not be imported.