The ticket sale starts some 6 weeks before each game. There is a special section on the website showing the exact dates:
http://www.werder.de/tickets/bl/verkaufsstart.php
In the "Bestellfrist" section of the table at the bottom of the page you can see the exact dates for each game.
On any of these dates, you can order the tickets of your choice online at:
http://www.werder.de/tickets/bl/heimspiele.php Scroll to the bottom of the page to access the order form (Bestellschein, German only). Once you filled in the order form correctly and sent it out, you will receive a confirmation of your order by e-mail (agaiin, German only). You can order up to five tickets per person per match.
What you need to know is that an order is no guarantee that you get tickets. If there are more orders than available tickets for the category of your choice, there will be a draw. All orders that have come in during the first three days of the sale period are taken into account in this draw. If you get tickets, the credit card mentioned by you in the order form will be charged (I assume bank account does not work since you are abroad) and you will receive the tickets. If you don't get tickets, you will be informed by e-mail.
You can always call the ticket center after any of these dates and ask whether tickets for a certain game are still available, but the tickets for the standing area are normally sold out after the dates in the "Bestellfrist", and there normally is a draw, so you can never be sure if you get tickets or not. Time between placing the order and knowing whether or not you get tickets varies; normally you should know some 2-3 weeks before the game.
I think, it will be even more difficult to get standing tickets next season since the "Ostkurve" will be put closer to the pitch (similar to the "Westkurve"), so there will be times with less places than normal.
If you're OK with tickets in any other area of the stadium, calling the ticket center works for many games (not Hamburg, Schalke, Munich though), and you can then place your order and collect the tickets at a specific counter at the day of the game.
BTW: If, at any time during the next season, you want to go to a specific game and have problems with the order form (which is in German), just send me a message & I will either try and help you with the form or place an order for you if it does not work.