Slowly and carefully! You should get together and have a coffee or something and a nice long talk. Do not under any circumstances have sex or alcohol (which could lead to sex). You need to have a talk about your feelings for each other. Are they real? What do you both want out of this? What caused the break up in the first place? If you decide you do want to start seeing each other again she will need to take some time to break if off with the new guy gently or you will all get yourself in a big muddle.
"The only thing to do with good advice is to pass it on. It is never any use to oneself." - Oscar Wilde