Slipper pattern

Hi to everyone reading this, I’m just trying to do a slipper pattern here and I can’t seem to understand how to do the instruction on increasing the stitches.
Slippers with 4mm needle cast on 21 sts and beg at back of heel.
[B]1st Row:/BP1,k1,p1, rep from to end.
[B]2nd Row:[/B]K1,p1,k1,rep from to end.
Rep these 2 rows to form single rib 8 times more.

Now here is where I get confuse…
[B]Shape Sides:[/B]
[B]Next Row:/B Cast on 18 sts, (p1,k1) 9 times over these sts, rib to end as set.

How do I cast on these extra stitches?
Please help, I appreciate any input with these thanks. :whoosh: