To get to Isinya town using Matatu from Nairobi CBD, then go to Railways Bus Station near Easy Coach and board matatu number 110 (Rembo Shuttle). One you get to Kitengela town, you will find many matatus headed to Isinya that you can board.
It will cost you about ksh. 200 (1 hour 40 minutes) for the entire trip using public travel means.