okay...here is how i teach mitered corners....
1. square up each corner with a large (at least 12" ruler)
2. now, this sounds weird, do it on a sample and trust me...... AFTER SQUARING UP...move the ruler to 1/8" away from the corner and angle down to the actual raw edge 3" down (shaves off a teeny dart shape)..
3. now repeat on the other side of the square...each 90 deg corner is now 1/8 off BOTH directions...
4. apply binding with the "seam down, stop, fold on 45, re-sew on the other side" method, lining up the raw edge of the binding on the raw edge of the quilt layer....
5. now when you turn the corner and all the layers are inside, they will be perfectly square...the tiny trimmed piece leaves just enough space for all the fabric inside...and i do not trim the binding...you need it inside to hold the shape of the corner.