1. The nature of the stitch means that the needle will go to the back at the corner point.
2. This is the same point you will begin the first stitch along the adjacent side - which means you would naturally un-stitch your last stitch.
3. If you skip a fabric thread or two, your corner will look open and your last stitch pull a bit funny.
So this is the trick...
1. After finishing the last stitch, turn your fabric over and slide your needle under the last stitch.
2. You can now re-emerge through the exact same point at the corner.
3. Stitch along the adjacent side and you have a neat closed corner.
Note how this corner point is looking a bit 'weak'? The width of the line kind of tapers at the corner point. This is great if you are stitching a sharp point such as the tip of a petal or leaf but there is a way to get the line of stitches the same width all the way into the corner.
1. Stitch to the corner. After the last full stitch, complete the line with at half stitch taking the needle to the back through the same hole at the corner point.
2. Turn the fabric over and secure the thread by sliding the needle under a stitch on the back.
3. Using the same hole in the fabric at the corner point, start the adjacent side with a half-length stitch, then emerge through the same hole one more time.
4. Now stitch the next side and your corner will look nice and solid.
Hope this was helpful,