Walk or find some other means to get to Serangoon Bus/MRT
Interchange.
Take 81 to Pasir Ris Estate and transfer to 89 and get down at
Downtown East.
81 and 89 share a few common bus stops in Pasir Ris, so keep a
look out for such a bus stop.
Alternatively, walk to the main Road (Braddell and Upp Sgn
Roads) and take 153 to Hougang Central Bus/MRT Interchange and
transfer to 89. This takes longer, but may be more convenient if
you are not familiar with transferring buses within Pasir Ris
Estate.
Thirdly, simply go to http://gothere.sg/ and plan your own
route.