be in bad relationship with Helen (starting choices), and get invited by Sarah in her flat for dinner in scene11 when Luke first meet her at the Hippocrates Center, and then get in bed with her and not kill her
1)At spaceport, choose "Luke misses Tanya" choice
2) Luke treats Helen badly (scold her, etc) during the next scenes
3) When Luke meets Sarah now will have an extra dialogue option, choose "though Tanya would be even more trouble"
4) accept Sarah's invitation
5) you then will have more choices but from now on is quite simple, the hardest part is to get the right pre-requisites in the beginning to unlock the dinner invitation