Casting on for return row of buttonhole

I’m following the standard way to make buttonholes where you basically bind off 4 stitches on row and then when coming back to the gap, you cast on four stitches. It suggests the single cast on method. I tried that and also the knit on method and can’t quite figure either out. Am I using the same yarn that I’ve been working with? Do I turn the buttonhole around to the other side while I cast on? I’m almost done with this hooded vest and would like to start a spring project!

Yep, you swap the needle to the other hand, use the knit or cable cast on, then put it back in the R hand to finish the row.

If you were using the single cast on (also called e-cast on, or backward loop) you would not turn the work but just knit or purl back to the little gap where you bound off the stitches and at that point throw 4 backward loops directly onto the right hand needle with your working yarn, then continue across the row.

I like to use a crochet cast on because it matches the look of the bindoff. So the top & bottom of the buttonhole look the same.

You bind off 4 stitches. The on the row back, crochet on 4 stitches. Use a crochet hook the same mm size as your needles.

This is also the way I make a provisional cast on. Easier for me that the crochet chain with knitting in the bumps.

Here is a link with this type of cast on.