Basically you fill it (Name, date and signature) and carry it around on your wallet, hoping this will be with you the day you pass away. You should also tell your family that you are donating your organs.
The website is at http://www.lahjaelamalle.net/
If you want one of these cards for yourself you just need to fill in your name and address at http://www.lahjaelamalle.net/fin/tilaukset/ (Google translation)
If you have a printer available use this PDF (cut the area in yellow, fill it and carry it with you).