In a simple text editor, declare the liga feature in aalt:
feature aalt{
feature liga;
} aalt;
Then program the ligatures thus:
feature liga {
# Latin
sub f i by fi;
sub f l by fl;
sub X X by XX;
etc.
} liga;
Then, SAVE AS your_file_name.fea. In FOG5, navigate to Element > Font Info > Encoding, then select the radio button for an external feature file, and browse for and select your feature file.