To get the corners nice and squared off, we will do 3 sts plus one picot each end.
Depending on how thick you want this border, use an sc, hdc or dc.. I will, however, give instructions using sc only.
The picot we use here will be the 3 ch picot – so ch 3 and then sl-st into the 1st ch (of the ch-3 just made).
(2 sc ; 1 picot ; 1 sc) in the next st [which marks one corner] ;
*sc in the next 2 ch and next sc ; sc + picot in the next sc ;
sc in the next sc and 2 ch, sc + picot in the next ch* ;
rep from *to* till the end,
then at the next corner rep (to) all the way around till you’ve reached the 1st st.
Join with a sl-st to the 1st st.