One of the facts of working in a pliable medium like fabric is that no matter how carefully you complete each step, there are bound to be little imperfections. If each unit is squared as you complete it, the final blocks will be nearly perfect when sewn together.
A 12" block that really is 12" and not 11 7/8" or 12 1/8" means that your quilt top will be flat. Your sashing or borders will fit as they should. The corners on the quilt will be square. There will be no nonsense about "quilting it out" when you find a bubble. Why spend a lot of time working around your mistakes when you can do it right in the first place?
Your square ruler makes this step a breeze. The diagonal line down the middle means that when you square something, the two parts are really HALVES. Without this line, it is possible to make a 4-patch be 4 1/2" but have one side be smaller than the other. This type of mistake can leap off the quilt.
Put a unit on the cutting board. Put your square ruler on it with the 1/1 corner at the top on your dominant side. You want the ruler to be as close to the edge as possible while still covering the two outside edges completely. In other words, you do NOT want to see any mat under the edge of the ruler. This block is being squared to 4 1/2".
Notice that the 4 1/2" line is well within the fabric on the two outer edges, the diagonal line matches the line in the block and the edges extend beyond the ruler. This is the part you will trim off. The top light square is exactly on the 2 1/4" mark.
Look at the interior of the ruler. This unit should be 4 1/2". You should see the fabric either ON the 4 1/2" lines or extending slightly beyond it. If it is too short, you have made a mistake. Figure out what that is now.
When squaring a half square triangle, the diagonal line will rest on the center seam line. The first two sides have been trimmed and the unit has been rotated to trim the second two sides in the picture below.
Second side lines up exactly on the correct ruler markings
Note: There is one difference between trimming a half square triangle and a 4-patch. You can still tell what the diagonal line should look like. It will go through the mid-point of the block. But as a safeguard, you should also have the ruler resting on the square in the proper size. In our 4 1/2" block, one square started life as a 2 1/2" square. We have sewn 1/4" into the seam allowance. The remaining square should be 2 1/4", right? That is how the ruler should be aligned.
If you have a broadband connection, you may wish to view this short video on squaring up a half square triangle.
Once your piece is quilted, you need to make the edges even and square before adding your binding. It is easier and more accurate to trim the quilt sandwich if the three layers are sewn together at the edge. Otherwise, the bottom layer can shift and you end up with edges that are not the same.
The absolute best way to sew the layers together is to use a walking foot and, with a basting stitch length, zigzag down the very edge of the outer border. Your stitches should be less than 1/4" wide so they will not show once the binding is attached. If you miscalculate, you can go back and remove them with a seam ripper.
If you do not have a walking foot (also known as an even feed foot), you will have to baste the edges by hand, right inside the 1/4" seam allowance. Once you have done this on a large quilt, you will run out to buy a walking foot!
After you have stitched the edge, the next step is to trim away the excess batting and backing. You have secured the three layers together so that they do not shift or fold under while you are cutting. They also will act as one piece of fabric when you sew the binding, which means you can use your regular foot and still have no ripples. It is easier to get a 1/4" seam allowance with your regular foot than with the walking foot.
Lay the quilted piece on your cutting board with the top and one side in position for trimming. Support the weight of the quilt if it is larger than your table so that it is not pulling at the edges. You have to decide what to use as a guide when cutting. If you have added a border which is narrower than the ruler, you can use the interior seam allowance as your guide. Line this seam up with the proper marking on your ruler and continue to place the ruler in that same position as you move around the four sides.
Put your 24" rotary ruler with the long side against the quilt side and the 6" width of the ruler running across the top of the quilt. This will allow you to trim around the corner and know that you are making a right angle. Trim.
Move the ruler down the length of your quilt. Align the ruler with your interior guide and overlap the previously cut edge enough so that you know you are still trimming in a straight line.
In addition to trimming off the excess batting and backing, you are trimming the edge of the top at the predetermined width of your outermost border strip, but quilting will often distort that edge and you will find yourself trimming off small pieces. The naked eye will not notice that the border has been trimmed. Wavy lines are a lot more visible.
As you get to the bottom, you will do the same thing with the square end of the ruler as you did at the top, making another perfect right angle corner. Continue in this manner all the way around the quilt.
When you have finished, check for any places where you have trimmed off the basting. Resew those areas. It is important that the three layers be sewn together so they will act like one piece of fabric as you sew on your binding.