After you decided on the signal that you want to measure, you need the complementary patch that enables the Sensor Dot to capture it. Byteflies patches are adhesive, disposable mounts that connect a Sensor Dot to the body and the Sensor Dot connects magnetically to the patch. Each patch is compatible with one or more roles which is explained here. Neither Patches nor Sensor Dots are currently waterproof. Do not shower or bathe with either!
When placing a Sensor Dot into a Patch, pay attention that the connection is magnetic and only one orientation is allowed.