Originally Posted by ckcowl
line the diagonal line of square ruler up with your diagonal seam...adjust the ruler so that a smidge is sticking out on one side and top making sure you have a smidge more than 5 1/2" showing on the other side and bottom...trim that first side and top; turn the block 90 degrees, line up diagonal again- line up 5 1/2" marks on the freshly cut side and bottom and trim the remaining side and top.
This is also how I do it - just finished 80 x 4" blocks for a border! I first read it on Quilt University, which explains quite well, with pictures. Under section called "Glossary" which is in alphabetical order.