To catch a Neeta Volvo Bus for Shiridi which would be the nearest boarding point from Mumbai CST
Most of their buses depart from Borivali in western suburbs and CST is in central line. However their bus goes via Sion (E). So you can board the bus at sion but do check with them before.
Ideally take a harbour line train Mumbai CST to Chunabhatti Railway Station. Though they mention the bus stop as Sion it halts opposite Highway Apartments (on Eastern Express Highway) which is closer to Chunabhatti Railway Station. You can take an auto rickshaw (with minimum fare of Rs. 18) upto the highway.