Here are the actual instructions on how it works and how you do it. You power meter needs to support the power meter profile under BTLE and/or ANT+. Fortunately virtually every current device does.

https://support.wahoofitness.com/hc/en-us/articles/115000369590-How-do-I-Calibrate-a-Power-Meter-with-the-ELEMNT-BOLT-