Simply pick up the red subway line, you go out at the station 'Alameda', swap for the green line and you quit at the station 'Baixa-Chiado'. Then you just need to walk 3 minutes and your're home!
In Santa Apolónia train station pick up the blue subway line and quit at the station 'Baixa-Chiado'. Then you just need to walk 3 minutes and your're home!