I like to press my top and borders and then lay the top flat to do the miter, 1 corner at a time. I leave extra border fabric and take the 2 “ears” and fold them underneath at a right angle to get a good miter. I press the miter in the border for a line to sew one. I pin the border so nothing shifts, sew on the iron line and then check that the corner lays flat. If it look good I trim the extra fabric away but I leave 1/2 seam allowances until all four corners are done. I iron again and lay the quilt top full out to check that it looks good. If I need to adjust anything, the extra seam allowance allows that. If it looks good I sandwich it.