There are two ways.
1. The easiest one is to take a taxi at the airport. Estimated costs are about 15 euros.
2. There is no direct connection between the airport and the hotel with public transportation. If you have time and you want to save some money using public transportation, the best way to reach the hotel requires three steps:
a. First, take the Aerobus at the airport (not the aeroshuttle, which takes another route). It costs 3.5 euros and takes about 40 minutes to reach Rua Aurea, where you should step out (see picture below). This is the last but one stop of the aerobus route.
[Top: Rua Aurea (the Aerobus stop is at the right of the picture). Bottom: Route to be covered on foot between the Aerobus stop and the stop of both, the bus nº 760 and the streetcar nº 25E at Rua do Comércio.]
b. Head for Praça do Comércio (a big open space along the Tagus river, see picture above) and before reaching it, turn to your left at Rua do Comércio (see picture).There, take Bus nº 760. Please, pay attention to name at the top (not only the number), as the same bus number covers the same route in opposite directions. The bus should read at the top Cemitério Ajuda. Alternatively to the bus, you may want to take the traditional Lisbon streetcar (locally called Eléctrico) nº 25E with the name Campo Ourique (Prazeres) at the top, which also leaves from Rua do Comércio. The price is 1.45 euros, although the streetcar is more slowly than the bus, and typically more crowed.
c. Step out at the stop called Santos-o-Velho (ask to the driver), which is close to the hotels' street. The Yorkhouse (As Janelas Verdes) hotel is 2 (3) minutes walking from the bus/streetcar stop.