ANSWER:
If skipping the fast was due to a chronic illness that doctors thought you would never recover from, you should feed one poor person for each day missed. But if it wasn’t a chronic illness, You have no option of kaffarah for your previous fasts. You have to slowly and steadily keep those missed obligatory fasts, try to keep every Mon and Thurs and three white days with dual intention, one being of your missed fast. However, if you skipped these days without any legitimate reason except laziness, you can’t make them up and it won’t be accepted.