your borders need to be longer than each side. place a dot at the exact 1/4" spot on each corner of the border (wrong side.
start and stop sewing exactly on that dot- attach each side--
turn quilt diagonally- line up the two corners on top of each other, draw a diagonal line matching the folded edge of the quilt, stitch along that line= trim corner to leave a 1/4" seam==
go to the tutorial page there are plenty of tutorials and pictures to help you along the way.
also visit Fons and Porter's love of quilting- they just had a technique show and have a vidoe on mitered corners. it does not matter if it is a binding, a border or a frame- miters are done the same way.