One of the hardest girls you will ever have to pick up in your life is an old flame, an old girlfriend, an ex. Whether or not she was easy to pick up the first time, second time around, you are going to have your work cut out and even then you may not succeed.
Therefore before you even try, you should be certain that you want her back as sometimes things happen for the better. If however, you are sure that she is the girl for you and no other girl replace her, only then give it a try.
Whatever the reason was that you broke up, you will have to apologize and if she thinks she is the one that played a part in the break up, let her know you truly forgive her. More often than not, part of the breakup will be due to you not changing your ways and so you must be prepared to show that you have changed.
I say show her because it is very doubtful that she will just take your word for it, you will have to find a way to convince her that you have changed. If she thinks the romance had gone from your relationship then you will have to reintroduce it.
This may mean taking her out for an intimate meal but you may have a better chance if you suggest a romantic weekend away for just the two of you. Perhaps to somewhere where you can take walks through the fields or woods, perhaps a sunlit walk on a beach.
If you really want to be adventurous, perhaps suggest a weekend away together in Paris, the city where romance was born. One of these, especially if you had never thought to mention them before, may win her over, convincing her that you really have changed and changed for the better.
Of course though you will have to see through on your suggestions and may have to find other things to keep the romance alive if you want to keep her.
Whilst on one of these promised rendezvous, you should never mention any part of the past you had together, not even the good times as that may cause her to remember some of the bad times as well so, instead of mentioning the past, mention the future and all the things you have planned for the two of you if only she would take you back.
I know this may be a lot but I did warn you that getting her back would not be easy and that you should only try if you were certain it is what you wanted.
If it is what you wanted though and all this works, you know it would have been worth it and so try not to let a breakup happen again as getting her back a second time is something you do not even want to hear about what all is needed.