The first step in assembling the sensor board is to spread solder paste on the pads where the smaller pieces will be added. In order to do this, we will be using a stencil (shown below).
First tape down a magazine or other scrap surface to work on.
Next, tape down two groups of sensors together in a right angle. These guide boards will help keep in place the group of boards you will be solder pasting.
Slide the sensor boards to be soldered into the corner formed by the taped down guide boards.
* Notice that in the pictures shown, we are solder pasting and assembling a group of 21 sensor boards. At any time, you can break apart the boards if you want to assemble fewer or if you think a step may be easier with the boards apart.
Next, place the stencil on the sensor boards - lining up the holes in the stencil with the corresponding metal pads on the sensor boards. Make sure you have the stencil flipped the correct way so that the smaller pads line up with the staggered holes.
When the stencil is lined up with the sensor boards, tape it down so it won't move around during solder pasting. It is best to tape down two adjacent sides to leave one corner of the stencil able to flip up to allow the sensor boards to slide out after they've been solder pasted (shown in the 5th picture below).