The start of any tunisian crochet involves casting on stitches, but sometimes stitches need to be added during a project that aren't attached to the prior row. Different cast-on methods are good for different situations.
This method adds stitches and give the appearance of a chain on the bottom. This method works well if a sturdy edge is needed. For this method you will need an additional hook of approximately the same size.
Slip Knot Cast-on
This method adds stitches that are stretchy. This method is good for adding stitches under the arms in garments because it is not bulky.