Here is a reference from the Winter League 2006 Site, on how to go to the Sengkang fields.

By Car:

Get into the TPE and exit via Sengkang East Rd/ Puggol Way. Be sure to turn the right way at Sengkang East Road, towards Sengkang Square.

You should be able to see the fields on your left from there.

By public transport from the north / north-east:

Drop off at Khatib MRT, and cross the bridge to the bus stop on the other side. Take 965, and it'll go to Sengkang Interchange.