After what you just did on the bind off row you have the piece in 3 sections. Looking at the smallest size (I don’t know which size you are making) those 3 sections would have 5 stitches, a space, 36 sts, a space and 5 stitches. The spaces will be where the leg holes will be. You are suppose to work with a separate ball of yarn in each of the sections.
So work across the 5 stitches with one ball of yarn, drop that yarn and take another ball and use it to work across the 36 stitches, then drop that yarn and take a third ball and work across the last 5 stitches. (One of those sections will already have a yarn you can use for it, but you’ll need a new one for the other two.)Then work back across using the yarns for each section when you come to them and dropping the last one used each time. You keep doing that for the depth of the leg hole. I think for the smallest size it was 1" or a little more.
Then on the row called the joining row you work across and cast on the same number of stitches you took off on the bind off row a little while ago. You would use a knitted on cast on (or a cable cast on–which is a type of knitted on cast on too). Work the first section as usual and then cast on the number you took off to start the hole, work across the middle section, then cast on again and work across the last little section. When you finish that joining row you will have as many stitches as you had before you did the bind offs for the holes, and they will all be on one needle, right next to each other like a regular row.
Does that make any sense?