2) One needs to take a bus or taxi to Lisboa Sete Rios Long Distance Bus Station.
3) From there one gets a "Rede-Expressos" ( http://www.rede-expressos.pt ) bus to Serta.
To travel from the Airport to Lisboa Sete-Rios Bus Station take the Aerobus Line 2 Bus from outside the main arrivals hall at the Airport. The service runs every 40 minutes from 07:30 to 22.00. Takes about 25 minutes depending on traffic.
One really need to get to the Bus Station 30 minutes beforehand to get you tickets unless one can manage to buy them online.
If you miss this bus, the next one is at 15:45 arriving 19:45.
http://rede-expressos.pt also operate return services.