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Mitering borders doesn't need to be as difficult as you thought! Give up those old complicated methods as I walk you through an easy solution to mitering corners on multiple borders.
The first step in the process is to decide how many borders you would like, i.e. inner border, middle, outer, etc. On a bed quilt, I often combine a 2 1/2" cut inner border with a 1" cut highlighting border (a color that sets the quilt off in a dramatic way) and finally about an 8 1/2" cut of my feature, outer border.
If your quilt top is 80" and you plan a 10" border all around, your 4 border sides will all need to be at least 100" long (a little longer is better). You will need to seam together enough lengths of fabric for each strip of the border to create the length that you need. When all your individual border strips are the length that you need, go ahead and stitch these individual border strips together in the order that you would like to attach them to your quilt. You should now have a border unit comprised of your full length inner, middle and outer borders, seamed full length.
Next, you will need to attach each border unit to a side of your quilt. First, find the center of your quilt (fold it in half and mark it with a pin), then find the center of your first border (fold it in half as well). Attach your border at the mid-point to the mid-point on your quilt. I usually do this process on my ironing board. From the center, begin to pin your border down as you work your way out to each corner.
When your first border unit is pinned in place, you can begin to sew. You will begin to seam the border on by starting 1/4" in from the corner of your quilt. Backstitch to hold that spot in place. Stitch down to the other end of the quilt and again, stop 1/4" from the corner and backstitch. This will leave 1/4" unsewn at each end. Your border units should hang out at both ends beyond the edges of your quilt (overhang should be at least as long as the width of the border). Repeat the process with the other three sides.
At this point, all four border units should be stitched to the edges of your quilt, with 1/4" unsewn corners at each corner. I now put the first corner on my ironing board. Try to lay the border flat, with room to manipulate the corners. Extend the top border along the board, past edge of quilt. The second border should be laid out crossing the first. Take that second border and fold it back (under) so that it extends (right sides together) with the bottom border. When folded in this way, your second loose border flap will be laying on top of the first, extending in the same direction. This fold will create your miter. You will adjust this fold until all strips in your unit miter nicely at the corners. When you are happy with your fold line, iron the fold down with a hot, steam iron. At this point, I use a wide piece of masking tape to lay across my ironed fold, down the fold line.
Now fold the border back so that you can see the crease line on the inside of the border corner. You will take the corner to your sewing machine and sew along the crease line (taking care not to stitch the tape on the inside of this crease- tape should be folded in half lengthwise down the crease). You will be stitching through both borders on a mitered angle that you will be able to see using the iron crease to guide you, and the masking tape to hold the corner in place.
You can fold your corner back to double check that your borders are still lined up. When you are happy with the finished product, pull off the tape and trim the seam allowance back to 1/4".
Now that your quilt borders are on, you're well on your way to the next step of basting & quilting your project!