Knitting chart help

Hello I have a pattern chart for mittens I am making. I’m a little confused because it says to take 13 stitches and hold them for the thumb but I don’t know when I am supposed to do this. Do I do the 52nd row and then take the 13 stitches after I knit them and hold them off?? ALso when I make the left mitten it says to do it as a “MIRROR IMAGE” will that go for the thumb as well as any decreases above that?? Please help. Here is the link to the mittens::

It says the thumb sts are underlined with the red line on the chart, so that tells you when. Does look like it’s row 52. Yes, the other mitten will be mirrored for the thumb placement too.

And yes, on round 52, knit across the thumb sts, place them on some scrap yarn and then continue the round.