Step 8: Adding the Flow Sensor
When you do add the flexible flow sensor, simply slide the metal square sleeves over the two prongs as far as they can go (pictured below). Make sure the sensor is oriented like in the picture; the white lettering should be right-side-up and facing inside (toward the photo sensor).
Then solder the prongs to the boxy sleeves on the flow sensor (picture 2). Fill in the gap where you can see the gold prongs in-between the silver sleeve making sure solder touches both parts. Make sure you don't bridge the two prongs with solder. Once soldered, you should no longer be able to slide the flexible flow sensor off the prongs.
The flow sensor is the last part to add - you've now completed assembling the Triple Sensor Board.