To start with, you’re looking for this multi-level chaos walkway that is outside the train station.
From inside the train station, head for signs that say “central gate” and the “scrambled” building.
From here you’re looking for the Shibuya Stream building, which can be seen from this walkway. Look for the sign I’ve circled in this image (there is a google logo at the very top of the building but that’s much harder to spot):
When you enter the building from any angle there are Google signs prominently displayed on most of the public signs with a circled G logo:
Google is on the 5th floor, up these escalators that are closed on the weekend but should be open on the weekdays:
