This is a desktop I am making which I want to add a breadboard end. Breadboard ends do 2 things. They hide end grain, and they can help keep a panel flat. However simply attaching a piece of wood to the end of a board will lead to disaster. It's very important to let the large panel move. A proper breadboard will control the movement, but won't restrict it.

Here I have the panel and the breadboard, with the tenon laid out

The general rule is the tenon length should be 2/3 of the width of the breadboard. My breadboard end is 3" wide, so my tenon is laid out for 2" deep.