This is a very simple achievement and is unmissable if you are doing this dlc (it can be done on any difficulty). Note the recommended level to do this is level 34.
To travel to the Duchy of Toussaint you must have completed the quest "A Poet Under Pressure". This can be done on your loaded save or on a fresh save. Alternatively you can start with the pre-made character for blood and wine only, with which you would start at the appropriate level and gear.
Once you have done this you should get the quest "Envoys, Wineboys". Follow this quest by going to a noticeboard in Velen and taking the new contract. After reaching and talking to a couple of Knights you will be on your way to Toussaint where you will get this achievement after the cutscenes are complete.