This will be a short tutorial on placing the borders on your drop shadow stack quilt. By now you have placed your sashing and your quilt top should look something like this:
Quilts tops can vary in size during construction based on your accuracy in cutting and the accuracy of your 1/4 inch seam. For that reason there is a method designed to help you kind of "square" things up. Here is a little video by Wenatqueens that will walk you through this process. This is an excellent tutorial so I'm not going to reinvent the wheel!
Once I have determined my measurements for the length of my border I start auditioning possible borders. I find it helps to take a photo of each so that you can kind of "stand back" and take a look at the different options.
One of the obvious choices is to border your quilt with the original fabric that you cut your stack blocks from. This is a nice option. Shown here with a thin strip of black. I did not select this because it took my eye away from the bold graphic nature of these blocks. Same for the reverse below.
What would it look like if you put the thin strip of the original fabric first and then a black border and used some of the smaller blocks in the border? Also interesting. Again, I did not select this option, although I was teetering (I love the little blocks in the corner) because it drew the eye to the border and away from the graphic nature of these blocks.
The third option I considered was a black border, some random extra blocks and a bit of the original fabric for the binding. This also did not make my cut...too much draw away from the graphic nature of the blocks.
After considering all of the options above, I decided to go with a simple black border, when I quilt this I will also bind it with black binding. Why? It keeps the focus on the blocks and the optical illusion of the drop shadow. I think you would agree that your eye stays there and does not drift to the border in this quilt. If there was a lot of busy stuff happening in the border you would be looking at that and the overall effect would be diminished.
Having said that, here is another one that I finished shortly after and you can see that I used the original fabric for the border. This quilt has a softer look, almost like an opal to me. That softer look did not require the boldness of the border above.
Let's talk a moment about the width of your border. The two quilts above use the same technique in creating the blocks. The blocks in the red quilt were made with a 6" square stack and the blocks in the opal quilt were made with a 4" square stack. The quilts are about the same width (65" vs 63") but the length is longer on the opal quilt. (65" vs 73"). Your border can be as wide or as narrow as you want, my rule of thumb that I personally use is to pick a border width that works with my blocks. Either as wide as the block, or 1/2 or 1/4 of the width of my block. I like the aesthetics of that balance. There is no "right" or "wrong" width to your border. You may need to cover the edge of a bed and don't want to make more blocks, you want them floating on the top of the bed, well then, make your borders long enough to complete your drop.
Quilting is an expression of you! Your choice of fabric, design, and ultimate execution is an expression of you. You don't have to color in the lines, the quilt police will not come to your door. Use your camera as an audition tool and don't be afraid to try something wild! Be bold! If you make one of these drop shadow quilts, I hope you will send me a photo, I would love to see your creations!