This stitch is worked in the same way as a normal crochet stitch, but is just worked around another yarn. You will need 2 different yarns to work this stitch.

Start your row with 1 chain stitch just in your casing yarn.

Take your centre yarn and lay it just above the stitches worked on your previous row.

Insert your hook into the first stitch on your previous hook, making sure you are going underneath your centre yarn.

Hook your casing yarn and pull through.

Complete your single crochet stitch as normal.

Continue single crocheting around your centre yarn, and that is how you work in your single crochet casing stitch.