The first step is to know if your ex still cares for you. Keeping her/him interested can only be possible if you have had a gentle breakup without the hostile drama.
Avoid communication for the first two weeks after the break-up.
Evaluate your personality and the reasons why she/he left you in the first place.
During these two weeks you need to change yourself, you cannot be the same person and expect to keep your ex interested.
After the initial two weeks, re-establish contact either by phone or by email.
See if she/he is still single, and if she/he has found someone do not be aggressive towards the other person because you cannot win your ex back this way. You were dumped because of him/her, so you now need to show that you are better than your ex’s latest choice.
Visiting your ex’s favourite pubs is another way to meet her/him and catch up on old times.
You need to show your ex that you are a changed person now. Remember you need to be the person she/her fell in love with which means some major changes in your personality should be in order.
Once reacquainted, plan a weekend together, if she/he is not ready yet, then take a step back and give her/him time to think over your proposal.
Try to avoid talking on the bitter memories of your relationship with the ex and focus on rebuilding the present.
Being friends after the breakup is the only option you will have, so be that friend for her/him who can lend a shoulder at any time.
Start flirting again to show interest and he/she might consider coming back to you and starting a new relationship.