Looking at these instructions you should be doing either a kfb or a pfb, depending on which one fits better with the pattern.
I came to that conclusion by the following method:
You will start Row 19 with 57 stitiches.
You need to add in 4 stitches evenly across the row to obtain 61 stitches.
Subtract the K1 at the end of the row from the original 57 and you will have 56 stitches to locate you 4 increases.
56/4 = 14
This means you will do 1 increase for every 14 stitches.
Because the instructions tell you to (Increase, P13) you will work the increase on stitch 1 of 14 and then purl the remaining 13 stitches in that set of 14, before doing the same for the next set of 14 stitches.