Hi and welcome to KnittingHelp!
There are several ways to do C5F. Your directions are different from the one that you found online. It's a split cable with a purl stitch running up the center. For the (p1,C5F): purl 1 (this isn't part of the cable), then slip the next 3sts to the cable needle (called stitch 1, 2, and 3), k2 and now slip stitch 3 back to the lefthand needle and purl it. It should be a purl stitch from a previous row. Finally, knit stitches 1 and 2 from the cable needle.
Can you provide a link to the pattern or give us the name of the pattern?