Here is a method of making perfect corners on collars, flaps, cuffs and anywhere else you need to make a corner. I have learned this method many years ago and ever since I always use it. It is very easy and simple and most importantly it works every time.
To demonstrate the method I am using a flap I made for a jeans jacket (where I still need to make button holes and put buttons).
The only thing you need is a thread about 50 cm (~ 20 inch) long, which needs to be folded 2 times for extra strength.
Sew until one step before the turn (as shown on the picture). Stop and put the needle in the lowest position. Lift the foot.
Now place the thread you have prepared earlier between the two layers of fabric. Pay attention that the thread is as close as possible to the needle.
Press the foot down and make one step with the needle over the tread. Leave the needle down.
Lift the foot and take the outer end of the tread and place it around the needle (the same as before).
Now both ends are together. Take them and pull them to the back, so they are not on the way of the needle.
Turn the fabric and press the foot down while pulling the ends of the thread away from the needle. Continue sewing over the stitching line.
I did all 3 corners of the flap in this way.
Trim the seam allowance and around the corners to reduce bulk.
Turn the flap, pull each corner out and iron.
And there they are all three perfect corners.
And just one more tip:
Do not remove the treads until the garment is finished. It makes it more comfortable when making the decorative stitches and ironing.
Let me know if you have tried this method, how did it work for you and if you have any questions.