Scanning is simple. TVs and boxes are pre-programmed to re-scan when told. Once the process has begun, don't press any buttons until the screen says the scan is complete.
Consult the owner's manual for your TV or converter box. The up/down and left/right buttons move you through the on-screen menu. Below are simple scanning directions for the most common converter boxes. Page numbers refer to the owner's manual that comes with your converter box.
Here is a guide for the most common converter boxes: