Mark 50mm from the ends and 25mm in from the sides, using an 8mm bit to drill holes for the cord.
Cut cord into two 1000mm lengths and thread through the holes to create loops, knotting as close to the ends as possible. Check the lengths are even.
Cut the remaining cord in half, threading each length through a carabiner and over a rafter or beam, tying the ends in a figure-of-eight knot (see diagram below) and pulling it tight. Clip the carabiners to the shelf cord loops, adjusting the knots to make it straight and checking with a level. Attach round and S hooks from which to hang the planters.