You can vote every 24 hours on the following sites. If multiple people play on the same internet connection they might be limited to one vote per day, however you might be able to bypass this by voting with a phone or tablet via the mobile internet.
Hint: If your mouse supports clicks on the mouse wheel (third mouse key), click with it on the buttons and they will be opened in background and save you half a second.
Open the link to the voting site.
Enter your minecraft name, solve the captcha and send the vote.
Execute the command /shop on the server where you want to redeem your vote tokens. If the credit shop is shown click the emerald to switch.