Pattern Confusion

[Kiwilicious]

HRi! I picked up a book yesterday, titled "I  taught myself to Crochet" by Boye. I'm on the first actual pattern, a coaster, and I'm pretty stuck. The instructions go:

Ch 14 loosely

Row 1: Sc in 2nd ch from hook and in each remaining ch across, turn -- 13 sc

Row 2: Ch 1, sc in each sc across, trn

Rep row 2 until coaster measures approximately 3.5 in.

Last Rnd: Ch 1, work sc evenly around all edges of coaster, working 3 sc in each corner, join with the slip stitch in first sc. Fasten off. Weave in all ends.

So I understood all the instructions up until the last rnd. I just can't understand what I'm supposed to do? What does it mean to work the sc round the edges, and 3 sc in each corner. Where does the slip stitch go? 

 

[bgs]

You have been working in rows back and forth until you have a square.  On the last rnd --note that it says round and not row.  This is your clue that now  you are going to work on all 4 sides of your square by going around.  You will need 3 sc in the corners so it lays flat and since you are going around you will get back to your first stitch and make your slip stitch into your first sc of the round

 

https://www.thecrochetcafe.com/tip-crochet-evenly-around/