Put the CD into the optical drive while Picard is running and then click “Lookup CD” and a little information box will pop up. It will either contain a release or numerous releases or be empty. If you are sure that one of the listed releases is the exact one you have, you can highlight it and it will open in the right panel of Picard. You can check it by clicking on it and then selecting “Lookup in Browser” on the Picard menu.
Often, different releases of the same album will (appear to) have the same disc ID as yours; or people will have entered their disc ID to the wrong release, so Picard might display a release that is not the one you have.
If clicking “Lookup CD” turns up an empty information box or none of the listings are your release or you want to be absolutely certain by examining the various releases before applying your disc ID, then click the middle button, “Lookup manually”. If your disc ID is already known to MusicBrainz, a browser tab will open showing the album your disc ID matches with, as well as a “Submit by the Simple Method” option. If you already are logged in to MusicBrainz, then you will also see 2 search boxes, “Search by Artist” and “Search by Release”. To do this properly (that’s why you’re on this forum, right?), you want to be logged in.
Copy and paste the release title (or type a variation that might turn up other possibilities—sometimes releases have titles in many languages or different spellings are used; a leading ‘the’ and other words may be present or omitted; subtitles might be present or omitted; artists’ names may or may not appear in the title, such as in “Ella sings Ellington”, etc) into the “Search by Release” search box and click enter.
Likewise, if you feel that “Search by Release” might not produce a match or produce too many matches; or you feel that you can more easily locate the release by “Search by Artist”, then follow the same procedure.
The browser will remember your disc ID (you can click previous and next page on the browser and it will still remember your disc ID, if you need to check back), so click search.
If an error is displayed (very confusing), then there is only one release that matches your search term and MusicBrainz already has your disc ID (this will be the album that was shown on the first page above the search boxes). This is confusing, but it appears that this is how the system informs you that it cannot add the same disc ID again. To be sure, retry this disc in a few hours.
Most of the time, though, you will be taken to a page that shows all of the releases that have release titles or artists that match your search terms from the previous step (if you did a “Search by Artist”, there will be an intermediate page where you have to select from a number of artists that are the same or similar to the one you entered).
You will notice that each of these entries has a dot beneath it (and, optionally, the disc number) and at the bottom of the list it has “Attach CD TOC”. Make sure you select, by ticking the correct button, the disc from the album you are submitting the disc ID for (sometimes more than one disc from an album will have exactly the same number of tracks). You will see that the release titles and artists are links, so you can open them in a new browser tab, if you want to check more closely. When you are satisfied, click the “Attach CD TOC” button.
If you either do not see your exact release listed, or you learned, by opening the release title or artist links that none of the releases shown were your release, you may add a new release. There is a button at the very bottom of the page to do this (I have never tried it).
What I find best to do is to open a new browser tab (while keeping the one with your disc ID open in the background). Create a new release (and release group, if necessary—the group will be created automatically, if there is not at least one release already). Once you are satisfied with the editing of the new release that exactly describes the CD you have, save it. Even though you have to wait 6 days for edits to be applied, it is already there!
Now, return to the background tab where you have the incomplete or unacceptable search results—the tab where your disc ID is (apparently) being remembered—and click the previous page button. Then, just run the search again, and this time you will see the very same list that you got before, but the new release you just created will also be in the list!
There you go: you can add your disc ID to the release you only created minutes ago… and you’re done (with that CD; make sure you submit a disc ID for each disc in the album)!