If you are talking about SIM on Bukit Jizhi Road, there are two relatively short routes:

Route 1: take the green line (east-west line) to the city hall, then turn to the red line (north-south line) to take the mrt to Newton, get out of the subway station, and take the bus 17 1, 66,67 to the opposite Nanyang High School. You can see the big sign of SIM when you get off the bus.

Route 2: Take the Green Line (East-West Line) MRT to Paya Lebar, and then turn to the Yellow Line (Loop Line) MRT to the Botanical Garden. After getting out of the subway station, take bus No.66, No.67, 17 1 and 170 to the opposite Nanyang High School Station. After getting off the bus, you can also see the big sign of SIM.