So for example with C6B you will slip 3 stitches from the LH needle purl-wise on to a holder and hold it to the back of your work like so...
You will then knit the next 3 stitches on the LH needle and finally you will knit the 3 stitches from the holder. C6B done.
For C6F you will do the same as C6B, but instead of holding the stitches to the back of your work will hold your stitches in the front like so...
You will knit the next 3 stitches from the LH needle and then knit the 3 stitches that are on the holder.
If you have trouble reading patterns here is a short video to show you how https://youtu.be/E-SN8fe5lD8
Thank you for stopping by and I hoped this helped you. x