You have to do three things:
1 – Repent to Allaah for this delay, and regret the negligence that has happened in the past, and resolve not to do such a thing again. Allaah says (interpretation of the meaning):
“And all of you beg Allaah to forgive you all, O believers, that you may be successful”
This delay is a sin, and repentance to Allaah from that sin is obligatory.
2 – Hasten to fast the number of days that you think most likely, and Allaah does not burden any person beyond his scope. However many days you think you missed, that is the number you should make up. So if you think it was ten days, then fast ten days. If you think it was more or less than that, than fast as many as you think, because Allaah says:
“Allaah burdens not a person beyond his scope”
“So keep your duty to Allaah and fear Him as much as you can”
3 – Feed one poor person for each day if you are able to do that. Pay all of that even if it is to one poor person. If you are poor and unable to feed another, then you do not have to do anything except fast and repent. For each day you must give half a saa’ of the staple food of your country, which is equivalent to one and a half kilograms for those who are able to give it. And Allaah is the Source of strength.