SewLibra:
Use 1/4 in. seams on your interfaced (one side) collar. Instead of pivoting, sew all across the top, then the sides and clip the corners close to but not through the stitches.
SewLibra:
Thread a sewing needle with a small eye and tie a double knot. You can knot it more if your fabric is thin. Leave a tail of thread beyond the knot, as you will need that later.
SewLibra:
Carefully reach inside the corner with your finger and insert the needle into the exact corner. Don't stick your finger with the needle!
SewLibra:
Pull the needle and thread until the corner pops out. If your knot comes through and the point is not quite right, make a bigger knot and repeat the process.
SewLibra:
Cut the needle off, reach inside and pull on your thread tails you left beyond the knot to pull out the remaining thread.