You have to do three things: 

1 – You have to repent to Allaah for this delay, and regret your past negligence; you must also resolve not to do such a thing again, because Allaah says (interpretation of the meaning): 

“And all of you beg Allaah to forgive you all, O believers, that you may be successful”

al-Noor 24:31 

This delay is a sin, so it is essential to repent to Allaah from that. 

2 – You must hasten to fast the number of days you think you missed, and (remember that) Allaah does not burden any person beyond his scope. However many days you think you missed, you have to fast that number of days. So if you think that it was ten days, then fast for ten days; if you think that it was more or less than that, fast as many days as you think, because Allaah says (interpretation of the meaning): 

“Allaah burdens not a person beyond his scope”

al-Baqarah 2:286 

“So keep your duty to Allaah and fear Him as much as you can”

al-Taghaabun 64:16  

3 – Feed one poor person for each day if you can afford to do that, even if you give it all to one poor person. But if you are poor and cannot feed another person, then you do not have to do anything apart from fasting and repenting. 

What is meant by feeding a poor person here is giving half a saa’ of the local staple food for each day; half a saa’ is equivalent to one and a half kilograms. 

Majmoo’ Fataawa al-Maqaalaat li’l-Shaykh Ibn Baaz, 6/19.