measure the part you want to surround-
then divide that by what ever goes into it---such as an 18" x 30" ---6 would go into---3x5---add 1/2" seam allowance and you need 15 6 1/2" blocks....
graph paper really comes in handy figuring out this stuff!
if the panal is not easily divided you could add a narrow border to bring it to a divisible measurement