Butt plug!!!

My bf and I went to the sex store yesterday and got a cute little butt plug for me. At the widest part it's about the width of his dick. I primarily want to use it to get used to having something stuck up there, because he loves anal and wants to try. I've tried before, and it felt ok once, but every other time it left me crying in pain. How should I use the but plug to "practice?"