You must make up for the missed days of this last Ramadan before fasting the 6 of shawwal.

After that, you must make up for the previous Ramadans.

I am fasting in shawwal but I have so many pendings about 2 months of fasting. So whatever I did in the month of shawwal will be considered my nafil fasting or the one is pending on me. As my intention is to complete all pendings by this year but I want to fast in 6 days of shawwal(as nafil fasting) also.