New to knitting. need to cast on in the middle of a project

I am new to knitting and am trying to knit a sweater for my toddler. I have completed a garter stitch for 6 1/2in and am now supposed to cast on 25 more stitches in addition to the 32 that I have been working. How do I cast on more stitches in the middle of the project??

The easiest cast-on for me to do in the middle of a project is this one:

Thanks. That was super easy!!