Ok, some day this will all make sense to me - but today I am really struggling.

I have the front of my quilt top done. I want to use 6 different fabrics for the backing and it needs to be a total of 63 x 82. I want to do them all like boarders for the entire back -but I don't even know where to start as far as diminisions.

I do want to do miterd corners so I know I need the strips to be long - that part is no big deal - but I can't figure out how wide they should be to get the effect I want. Can anyone help?

Based on your example
Use the following as your guide if all borders are the same size:
1st border (outside) 3" (cut 3 1/2)
2nd border 3"
3rd border 3"
4th border 3"
5th border 3"
All borders equal 15" X2 (top and bottom or two sides) equals 30".
Center block is 63"-30"=33" for width and 82"-30"=52" for length.

If you want different width borders just fill in your numbers and do the math. Don't forget to add seam allowance before cutting (ask me how I know, lol)
Good luck

Thank you!!!

And I would cut the last border at least 6" wide (after including the seam allowances on all the previous borders).
That would give you some "working fudge" factor.

You're talking about doing a miter that's about 22" long.

You need to make very sure that you measure exactly when making the border strata - no extra fullness in the fabric. Otherwise, you'll have borders with the topography of the Rocky mountains. ;-)

Maybe I won't miter the corners! :)

You have to add seam allowances when you cut them. Now that is where most people make an error And if you have a longarmer quilt it it needs to be wider to accommodate putting it on the machine