Knitting a placket with circular needles -help please!

Knitting a toddler jumper from 40 year old pattern book. I’m doing OK in rounds with circular needles until this place where the “placket” begins.
Directions say - at beginning of a round: “PLACKET: Turn and cast on three stitches for underlap, K 3 sts, P to end of row. Work back and forth in stockinette stitch, keeping 3 sts of underlap in garter st, until desired length.”

Problem areas -1) I don’t know what a placket is supposed to look like. I think it should be an opening (that probably will button) on the side of this circular skirt, so the child can get into it.
2) What does ‘turn’ mean? Reverse to the pearl side? I have tried to do that and cast 3 stitches onto left needle where the wool is. But if I try to then knit this with right needle, there is long string.
3)Can’t figure out how to knit back and forth in the middle of this garment (I do know where my row begins) and make a placket. I don’t know what this is supposed to look like because the illustration does not show this part.
Any help greatly appreciated.

A placket is exactly what you surmised. Here are some pictures of sweaters with plackets. It’s the opening at the neck that interrupts the knitting on the front and contains the buttonband (underlap) or the buttonholes (overlap). You’re going to knit the same thing but at the side of the skirt.
Yes, turn means reverse to the purl side. Turn the knitting the way you ordinarily would when you knit back and forth in the flat. So, at the the beginning of the round marker, take the needle that’s in your right hand and has the working yarn hanging down from it into your left hand. Cast on 3 sts using a knitted or cable cast on (or even a backwards loop wiil work here). Leave the needles right where they are in your hands, needle with cast on sts in your left hand with workng yarn hanging off it. Now knit the 3 cast on sts and continue purling the rest of the sts until you get to the small gap at the beginning of round (where you cast on the 3 sts). Here you turn again so you’re back on the knit side with the working yarn hanging off the needle in your left hand. Knit all the sts including the 3 placket sts that are at the far end of the row.
If it helps, you can think of knitting back and forth as alternately purling on the inside of the tube you’ve been making and then knitting on the outside of the tube.
There are videos for knit or cable cast on under the Free Videos tab at the top of the page, Cast Ons.

Thank you so very much for your speedy reply! It is extremely helpful. I am still not sure what happens to that connecting piece of yarn but will cross that bridge when I come to it. Can’t tell you how much I appreciate this help.

I think if you take it step by step you’ll see that there isn’t any connescting piece of yarn. The cast on sts are right next to the first sts on the left needle and the yarn is coming off the end of the cast on sts, just where you need it to begin knitting.
Good luck with the skirt which sounds like a very sweet idea.

Yes it did become clear as I knitted that this will stay open at the sides. My only problem may be those three cast on stitches that are supposed to be knitted as garter stitch. I have watched the videos over and over but I seem to have a deficit in this area. I think this overlap which I believe gets turned under will not be too important visually. I am encouraged that with all this help I will become a great knitter.
So thanks again for your support. This jumper should be very cute. Next step is the little bib top with straps and another go at the Faire Isle design that goes around the bottom of the skirt.
THANKS again (can’t find that button) and Happy New Year!