How to get there and location of the Mirador de San Nicolás?
If you are looking to do things efficiently and inexpensively, and in a way, as the locals do, then your best option is to take public transportation. There are two buses that arrive at the Mirador de San Nicolás: C31 and C32 and both lines go through Plaza Nueva, Paseo de los Tristes and Carrera del Darro in the center of downtown Granada. The C32 also has stops at the Alhambra and the Realejo neighborhood, so you can hop on either of these to go to the Mirador de San Nicolás. You can also take Red Alhambra minibusses parked at the viewpoint. Also, the C34 leaves from Plaza Nueva, which is also a minibus, and goes to Sacromonte. Sacromonte.
Just as a heads up, these buses tend to be on the small side and only carry about 20 people. They usually are packed and you might have to watch several drives away, filled to capacity before you can get on.
The C34 bus tends to be less crowded but you have to get off when you see the statue of gypsy Chorrojumo, in the middle of the Cuesta del Chapiz, just before the bus turns right at the Placeta del Peso de la Harina.