There are three ways of sewing corner hems on slits or on edges of fastenings/closures.
Sewing corner by hand: first sew the actual hem, then turn under and baste the hem allowance on slit or fastening facing. Press and hand-sew onto hem. If the fabric is very thick, previously trim the facing and hem allowance to about 1 cm.
Over-casting corners: Mark the required length. Neaten/finish hem allowance and/or facing allowance of slit or fastening. Turn facing or seam allowance outwards and stitch in place along the marked hem line. Trim allowances to 1 cm. Baste slit or fastening/closure edge and hem allowance inwards, press edges, then hand-sew hem. Hand-sew facing or allowance onto hem.