Keep in mind that only Japan gets the card sleeves, but you can still purchase them from overseas.
For details about the card sleeve contest, see this article.
The process of voting starts at https://www.db.yugioh-card.com/yugiohdb/. From there, you need to log in with your KONAMI ID.
(If you play Yu-Gi-Oh! Duel Links, but have not backed up your in-game data yet, you definitely want to register an ID so that you can back-up your data as well. After you register, just tap the gear-shaped icon in the top-right corner of your game screen in Duel World, and tap the button that says “Data Transfer Settings”.)
After you’ve logged in with your KONAMI ID, you’ll need to click on “Card Search”.
Then, search for your favorite card. Once you’ve navigated to your card’s page, select the “日本語” option in the language settings in the top-right corner, like so:
Now, all you have to do to vote is simply clicking on the black button as indicated in the image below:
And now your vote is cast. You only get 1 vote, but you can change your vote at any time by going to another card page and clicking the black button there. Only your most recent vote will count.
To change your language settings back, just go up to the top-right corner of any screen and select your language.
If you want to vote for a currently OCG-only card, you’ll have to find the Japanese card name and copy/paste it into the Japanese card database to find it.
Check our previous article to see what the contest’s current top 12 cards are (at the time of this article’s publication).