Quilters make a number of artistic decisions when designing a quilt, one of which may be whether or not to sash it. Sashing is what goes between the blocks in a quilt. Not all quilts have sashes, but they are useful in a number of ways. They isolate quilt blocks from one another, which can be visually pleasing if the blocks have disparate colors or designs. Sashes also increase the size of a quilt. The size of the sashes you would use would depend on the dimensions of your quilt blocks.
Measure your quilt blocks. If they're small, use narrow strips of fabric for the sashes. Larger blocks can accommodate wider sashes. For example, a narrow two-inch-wide sash would complement six-inch-square quilt blocks, but a four-inch-wide sash would dominate blocks of that size. However, 14-inch-wide blocks would go well with four-inch-wide sashes.
Choose a color or pattern that complements the colors of your quilt blocks. If the blocks are very busy or multicolored, a solid-color sash may be the best choice.
Lay out your quilt blocks in a trial pattern. Rearrange them as necessary until you're pleased with the result.
Decide how wide you want the sashes to be. For instance, assume that the quilt blocks are 10 inches square and you want to add two-inch-wide sashes. In this example, the quilt has four rows of three blocks each.
Calculate the length that the horizontal sashes need to be for each row by adding the dimensions of the three blocks and the sashes you will be inserting. In other words, there will be a sash on the left edge of the quilt, then a block, a sash, a block, a sash, a block, and a sash on the right edge of the quilt. Add the widths of each sash and block: 2 inches plus 10 inches plus 2 inches plus 10 inches plus 2 inches plus 10 inches plus 2 inches equals 38 inches. Add 1/2 inch to this length to allow for a 1/4-inch seam on each end. Therefore, each sash should be 38-1/2 inches long.
Cut the sashes 2-1/2 inches wide to give yourself a 1/4-inch seam allowance on each side. Cut five sashes, since you have four rows of blocks, and a sash will go at either end of the quilt as well as in between each row of blocks.
Add your horizontal sashes to your quilt layout, positioning the quilt blocks where they will be on the finished quilt top. Observe that there are vertical 10-inch gaps where the vertical sashes will go. Counting the gaps shows that you need to make 16 vertical sashes.
Cut the 16 vertical sashes 2-1/2 inches wide and 10-1/2 inches long, again to allow for 1/4-inch seams on each side. Place these sashes in the gaps of your quilt layout. All the pieces that you need are now there.
Pin the vertical sashes to the sides of each quilt block. Sew the pieces together. Press the seams toward the darker fabric.
Pin the horizontal sashes to your newly completed sash/block rows. Sew them together. Press the seams toward the darker fabric.