Start with a small mitered corner application to gain confidence in making excellent mitered corners every time!
To start gather notions and fabric scraps to practice :
My square measures 8 1/2 inches, and the 4 strips are 1 1/2 x 12 inches. Take special note here: when making mitered corners your strip length needs to be much longer than your square, the wider your strip, the more length you need. I don't have a formula here, but when in doubt make it longer :) When I was doing this the first time on my tree skirt, I ended up making my first round of border half the width to accommodate the miter -- it wasn't long enough for the width.
On with the instructions:
pin your strips, center on your square, on opposite sides
Sew with 1/4" seam -- I don't reinforce my seams at all, it may be helpful to pull out a few stitches when setting the miter.
I press open here to cut down on bulk
pin and sew opposite sides borders to the square
Press open again
Fold your square on the diagonal, this works for rectangles too, but you would need to do one corner at a time.
Match up your seams, making sure that the corner is lying flat without excess, and pin
This is how I pin, you don't want the seams to move, and you need your tails to be secure as well
You can do both corners at once, just flip it over for the opposite corner
Next line up your ruler on the diagonal, and mark the sewing line on the tail, like so --
And sew on the line that you drew
Open up your square to check your corner
If you are pleased, trim to 1/4" seam allowance, and press, if it's not quite right just pull out your stitches and try again. Take your time lining things up :)
Now go back and fold in the opposite diagonal and repeat
I like to trim up the extended seam allowances
You should have 4 of these!
So far I've used this technique for my tree skirt, and a few swap blocks. I envision throw pillows on the couch as the perfect project for mitered corners. Maybe that will be next :)
I also made a downloadable pdf version of this tutorial available here.
I think that's everything! Let me know if you have any questions and I will do my best to answer them :) Have a great weekend!