How to redeem CD/BD/etc serial codes and get reward items in SIFAS.

WARNING: According to WW Support, serial codes are officially unsupported on the WW version. While you can receive serial code rewards there as of now, this could change at any point, and you'll get the items only on your JP account then.
UPDATE on WW Accessory Availability: Apparently, WW has suspended adding Accessories past the Anime Solo CDs. No one has received the Nijigaku Piano or any other Accessories after it.

• The SIFAS account you want to have the reward items on, linked to a SIFID.
• A SIFAS JP account linked to the same SIFID.
• A valid SIFAS serial code.

1. Login to your account on SIFID.net.
2. From the home page, go to Account Management > Change Registered Information.
3. Change your account's language to Japanese (日本語) and hit Confirm twice.
4. Return to the home page by clicking the SIFID logo (スクフェスID) at the top.
5. With the language set to Japanese, you can now go to the Serial Code entry in the menu (シリアルコード).
6. Find the campaign you want to redeem the serial code for. They're ordered chronologically, so if you have a serial code from a recent release, it should be fairly high up. It's easiest to compare the label on the CD/BD with the text on the links. Alternatively, copypasting the site into a translator might point you in the right direction.
7. The next page is the Terms and Conditions of serial code redemption. At the bottom of the text, you can find a listing of the items you'll receive. Compare these to the items printed on the paper with the serial code to make sure you got the right campaign. Click the pink button to continue.
8. Enter your 16-character serial code in the input field, without the hyphens. Below the text entry, you'll find a text box with your account's information. Check the name and friend code there to make sure you're indeed registering the serial code to the correct account, then hit the orange button to redeem.
9. While the site says it might take hours for the items to arrive in your present box, it usually appears within minutes. If it doesn't show up even after waiting, the site recommends checking the SIFID linking menu in-game to force a synchronization.
10. Don't forget that you probably want to change your language back: return to the home page again by clicking the SIFID logo (スクフェスID) at the top, then go to Account Management (アカウント管理), and Change Registered Information (登録情報変更). Find the dropdown for language (言語), select your original language, then hit the orange Confirm button (登録) twice.