You may want to start by apologizing to your ex depending on the reasons you broke up. You will also have to give up on the things that broke you up and start by being friends. Get to learn more about your ex and the likes and dislikes and give your ex a good amount of space and do not put more pressure on it.
The best way to get an ex to come back to you is by trying to become good friends with them. If they have a new gf support him and make him believe you are happy for him. Talk to him all the time and be crazy nice. If you guys become comfortable again and build a friendship, he may realize he misses you.