Your file should generally look like this portion of a full feature file:
# --- kern classes
@A_LC_LEFT = [a aacute acircumflex adieresis ae agrave aring atilde ];
@A_SC_LEFT = [AEsmall Aacutesmall Acircumflexsmall Adieresissmall Agravesmall Aringsmall Asmall Atildesmall ];
@A_UC_LEFT = [A Aacute Acircumflex Adieresis Agrave Aring Atilde ];
# single pairs
pos A quotedblright -129;
# class kerning
pos @B_SC_LEFT @A_SC_RIGHT -19;
pos @B_SC_LEFT @B_SC_RIGHT -21;