Once you've registered for an ORCID iD, you should authorize CrossRef Metadata Search to link to your ORCID record.
To do so, first log in to view your ORCID record. Your ORCID record should be the screen you see upon logging in; you can visit it directly at https://orcid.org/my-orcid. Scroll to the bottom of your ORCID record to the "Works" section. You may not need to scroll down if you haven't added entries to other parts of your ORCID record!
Hover over the "Add works" dropdown menu to the right and select "Search & link:"
You should see a list of options to link works. Select "CrossRef Metadata Search:"
Finally, select "Authorize" to allow CrossRef Metadata Search to link to your ORCID record:
You will then be redirected to a Crossref search. You may easily add any works to your ORCID record by selecting the "Add to ORCID" option:
You'll only need to authorize CrossRef Metadata Search once. CrossRef will monitor newly published works, and, if it finds a work it thinks is yours, will send a notification through ORCID to ask you if you'd like to link the work to your ORCID record.