If you are driving from Stockholm take highway 73 for 37.5 miles. This takes you to Nynäshamn . Take the turn at Telia Road / Nynäsvägen and this leads directly to the ferry terminal.
If you are driving from the west coast or from southern Sweden, take route 225 between Södertälje (E4) and Nynashamn (about 31 miles). Drive through Nynashamn to Nynäsvägen and take the first right turn on your right, about 150 yards and leads to the Stena Line Ferry Terminal. Also, you can continue driving on the E4 towards Haninge and then take route 259 from Haninge to Nynashamn. Even thought this route is 12.5 miles longer, it is a better way to drive.
Please note that check-in takes place in the cargo terminal, therefore; you need to follow signs for freight vehicles.