How to made increase
 

(WS): *P2, p into front and then back of next st (increase made), p1, p into front and then back of next st, rep from * across to last 5 sts, p 2, p into front and then back of next st, p last 2 sts – 99 sts at end of row.

Thanks

Here's a video of pfb which is much better than any explantion I could manage!

Purl into the stitch as normal, but leave it on the needle, then purl into the back leg of it.