no1) take a bus from the cosway to the nearest kranji MRT station (only 90 cents in RM) but i forgot what bus number but u can ask people there
no2) when reach kranji MRT, ride to harbour front MRT station which is located at Vivi City inside (going to take u 45-50 mins to reach there from Kranji)
no3) buy monorial ticket from vivo city sentosa ticket counter
no4) ur at sentosa island already and find ur way to USS

yes...harbour front is an underground train station located underneath vivo city.