There are a few options for attaching your Circuit Playground Slouch Detector to your garment. Some other items, like the JST extension cable, can also make locating the battery more convenient.
The silver pin with adhesive foam back can be used to attach the Circuit Playground to most any garment.
Remove the protective film from the adhesive back and apply the pin to the back of the Circuit Playground.
The magnetic pin is a nice option if you don't want to stick holes in your shirt. Or just don't like sharp pointy things.
Remove the protective film from the adhesive back and apply the metal strip to the back of the Circuit Playground.
Just to be safe, apply the pin at the angle shown. It is metal and if you put it on horizontally, there's a slim chance it could short the 3.3V pad on one side to the GND pad on the other.