Are you using plain or patterned fabric?
Plain is easier.
One trick to make the miter work on patterned fabric is to use a mirror to see what it will look like first.
Take a strip of the fabric you're going to use, and place a straight edged mirror on the fabric at a 45'angle.
You'll see in the mirror what the corner will look like if you cut the fabric at that point. You can move the mirror along the strip of fabric until you see the result you want. Then cut the fabric longer than that point by at least as wide as the border is.
When you have the second strip of fabric, fold the end UNDER at a 45 ' angle facing out. Press it there. Then place it on top of the other piece until the corner looks like you want it to. Pin the underneath layers together and unfold the top layer. Sew along the pressed line.