Adding borders aka Window Sashing
First, measure the width and height of your wallhanging via the center of the quilt top. Now, we will add the lengthwise borders first. Take the length of your quilt top, and add 3". You will need two strips, one dark, and one light, of fabric at this length x 2.5" wide. For example, 29.5" x 2.5". (If needed, you may join two pieces of fabric together to get this length, but sew them together using a diagonal seam.)
Next, cut a strip of fabric using the same fabric as your "dark", the measurement of the width, plus 6" x 2.5" wide. For example, 20" x 2.5".
*Note, when following the steps below, make sure that you have the border fabric in place in the position that you will sew it. That is, your markings in the steps below will need to be placed at the end of the border as it attaches to the bottom of the wallhanging. You may wish to actually place your borders in place, the excess fabric extending over the bottom of the quilt top, before following the steps below.
We will add the side borders first. Follow the steps below for adding both the dark and light borders.
- Using a marking pen/pencil, measure in 3" from one end. Now, measure in an additional 1/4" from that 3" marking, from both the horizontal and vertical sides, such as:
and mark that with a dot. Make sure that it is visible, you will make use of this later. - Now, using your 3" marking, fold the border in half, that is, fold one end in half til it meets the 3" marking. Make a nice finger crease, then fold in half again, just to the 3" marking, and finger crease both sections. What you are doing is creating markings that you will use to match up with the quilt top.
- Repeat the steps above to mark the second border.
- Fold the quilt top in half, horizontally. Pin or finger crease to mark, on both sides of the quilt top. Now, fold in half again, thereby creating 4 increments, and pin or finger crease to mark.
- Match up the creased/pin markings for border 1 as it is attached to the quilt top. Use additional pins if needed to ease any extra fabric. Now, starting at the 1/4" marking, sew a few stitches, backstitch, then continue sewing to the end of that strip.
- Repeat the above to join border 2 to the quilt top.
- We will similarly mark the bottom border. First, measure in 3" from each end of the border strip, and mark a line with your marking pencil. Now, mark another 1/4" dot, to indicate 1/4" from the 3" marking, as well as the edge of the border strip, just as you did for the first two borders.
- Fold the border strip in half, mark with a finger crease or pin. Fold in half again, but, only until the ends meet the 3" markings. Finger crease or pin both sides of the center fold.
- Fold the quilt top in half vertically. Mark with a pin or finger crease. Fold in half again, pin or finger crease both sides of the border to mark.
- Match up the markings on the quilt top with those on the border, pin in place. Pin additionally to ease any excess fabric.
- Starting at one of the 1/4" markings, and taking a couple of backstitches, sew the border in place to the second 1/4" marking, and take a couple of backstitches.
- Now comes the tricky part. Refer to the pictures that follow:
Make note of the location of your 1/4" marking. 
Fold your quilt top so that border1 overlaps evenly with the bottom border. 
Using a ruler, extend the line created by the diagonal fold onto the back of the
border1 fabric. Using a marking pencil, draw a line on the border1 fabric.
Make sure that you can see the line clearly.
- Next, folding the seam allowances of the border1 and bottom border fabric out of the way, find your 1/4" marking, and starting at it, taking a couple of backstitches, sew along the line that you just drew.
- Open the fold and check your seam before trimming away the fabric. It should look like this:
- Trim the seam allowance to 1/4", press the seam OPEN, and trim any excess fabric.
- Repeat the steps above to add the second border.
(*Note, if you want to add a "ledging" to your window, add only the two side borders at this time, then skip down to the "So, you want more of a ..." section. )Now, use another shade of brown, and add a 3" border, all the way around the quilt top. Add to the two sides first, then to the top and bottom. Remember to measure your width and height via the center of the quilt, and fold and mark the borders and quilt top in 4ths, match up the markings and pin, to evenly distribute your border as it attaches to the quilt top.
Your finished top should appear similarly to this:
You are done!
You are now done with the quilt top. Sandwich it, quilt it, and bind it as desired.
So, you want more of a window ledge than the example...
Simply repeat the steps above for adding two side borders and a bottom "ledge" border. The two side borders should be of fabric to represent your wall next to the window. The bottom border fabric can match either the first sashing fabric, or the outer border fabric. This will give you that platform kinda look that you find in older windows. It will be large enough that you can applique on a vase or something to accentuate the ledge look of the window. For the side borders and ledge, use a 3" wide fabric. Remember to add an additional 3.5" to the lengths of the two side borders, and an additional 7" to the width of the bottom border. You will then need to add a 3" border to the top, and a 2" border to the bottom.
Sandwich it, quilt it, and bind it. And you are done. :-)