When I first started quilting, I didn't understand the function of borders. Sure they look great and add to the overall beauty of the quilt, but there are also a couple specific reasons for adding them.
When all the pieces have been sewn together in the center of the quilt top, most designers add one or more borders. The first border is the most important because it can keep the edges of the quilt top from stretching. Structurally, its main purpose is the secure the edges of the quilt.
Square up the quilt before adding any borders. Make sure the sides are equal in length and the tops are equal in length.
There is a correct way to sew borders onto a quilt. Let's add the right- and left-side borders. Fold the top in half along one of the sides. Measure it and multiply by 2. Place a pin in the fold. Cut your border fabric to the width required by the pattern and the length of your quilt. Fold the border strip in half and put a pin at the fold. With right sides together, match the center pins and pin together. Place another pin at each of the edges. Then I lay the top out from the center pin to one of the edges and pin that section. Then I lay the top out from the center to other edge and pin that section. Ease and stretch the fabrics as necessary, if necessary. You don't want to distort the center part of the quilt. Repeat for the other side. Adding a border in this way prevents you from distorting the quilt and keeping it to its measurements.
To sew the top and bottom, repeat the process.
What you don't want to do is start pinning your border from one edge to the opposite edge. This can distort your quilt top and make it wonky. Take the time to sew on the borders correctly. The end result is worth the effort.
Repeat this same process for all the borders you add to your top.
Your quilt top is now finished!!
Making a quilt sandwich
The very definition of a quilt is the joining of three layers of fabric together. Like a sandwich, you have the bread (the top and backing) and the filling (the batting). If you send out your quilt to be quilted, provide the quilter with the top, backing fabric, and batting. The quilter will often have specific requirements for how much larger the batting and backing should be. Most often they are about 3-4" inches all around the quilt top. For example, if the top is 20" wide, the back and batting should be 28" wide, which allows for 4" on either side. If the length is 30", cut the back and batting to 38".
Quilt as desired.