Single crochet each row(on the first row ,crochet your first stitch into the second chain)until the item is square, about 18 rows.
Switch to contrasting color. Half double crochet around the entire border.
Work in loose ends.
Repeat until the desired number of coasters are complete.
The pictured example is displayed in a paper mache box. For a simpler way to complete the project as a gift, make a long chain stitch in the border color and use it to tie together the coasters in a bow.