The do it in the shower and there are youtube
videos on how to do it. Or you can do it when you lay in bed. Basically it is just feeling around for any lumps or changes and if you do it enough, you notice these things. You can also ask your doctor next time you see them. At your age, I had a lump show up out of nowhere. I went to the doctor the morning I found it. It was a change. (turned out to be a fluid feel cyst they asperated).