Follow the signs for transit/T2E and look for Acces No 1 = SkyPriority signposts. Some of them have poles that form a square over the entrance with a banner that IIRC is orange and purple on white. There will probably be an airport staff member, IIRC wearing an apricot colored jacket, verifying credentials (SkyPriority on your boarding pass for the CDG-BOS flight, try to do OLCI to make sure that you have a boarding pass in case LIS can't give you one) for access to the special line. Also be sure to check the gate before heading off to L or M.