To figure out where to put the buttonholes you can count the rows on the other piece you made and put the marks where you wanted the buttons. Count the number of rows from the bottom to where the buttons begin and the rows between. Say, if you have the button marked at the 30th row when you get to that row begin your buttonhole.
1st buttonhole row (right side) K2, k2tog, y2rn, skpo, k10, k2tog, y2rn, skpo, k to end, then
2nd buttonhole row: Working twice into y2rn, p to last 2sts, k2
So, I get that that is the top and bottom of the button hole, right? What I don't know here is: What a y2rn is and what it means by working twice into it.
It takes the two rows to make the button hole. When it says to y2rn, it means to do what we call a double yarn over in the States. Then on the next row there will be two yarn over strands where you did that maneuver. When you get to them on the purl row you need to purl into each of the strands. I suggest you purl into the first one in the front like a regular purl, and into the second one do a purl through the back loop.
The double yarn over replaces the stitches you lose when you do the k2tog, and the skpo. You lose two and the double yarn over adds them back so you keep your stitch count.