How do I drain my deep cycle batteries?

I have a query regarding my deep cycle batteries. I don’t understand any terminology of electronics. You can say that I am ‘Electrically Challenged’ person. One of my friends told me that it is quite important to drain deep cycle batteries before charging them to full charge. So I want to know about the method or a way to drain deep cycle batteries? Can I use booster cables or something else to properly drain deep cycle batteries before recharging them? Waiting for all the relevant information pertaining to my query. Thanks.