How does a worshiper perform the 'Isha prayer after remembering it was missed while he is leading the Fajr prayer in its second rak'ah?

If you remember during Fajr prayer that you have not prayed 'Isha, you should complete your Fajr prayer, then make up the 'Isha prayer. It is recommended for you to make up the Fajr prayer after 'Isha to maintain the proper sequence.