The following is directly from your pattern for the slippers
ROW 1: P 1, *k1, p1; repeat from * to end. <==this is ribbing
ROW 2: K1, *p1, k1; repeat from * to end. <==this is ribbing
Repeat these 2 rows of ribbing for 1 1/2 (2-2) ins., end on wrong side.
First DEC. ROW: Work ribbing for 7 (7-9) sts, * slip, k and psso, k1, k2 tog. *, work 3 (5-7) sts ribbing, repeat between *'s once, finish row in ribbing; 23 (25-31) sts.
NEXT ROW: Work 7 (7-9) sts ribbing, p3, work 3 (5-7) sts ribbing, p3, finish row.
So, for the first decrease row, you do the ribbing as established for 7 sts (the first row would be P,k,p,k,p,k,p), then follow the rest of the instructions on that line.