You must have perfect bias for the covering. Paula Milner offers directions for Continuous Bias Strips. A few helpful products are offered by Nancy's Notions - Enter the catalog. Although I haven't tried the products, they appear to be extremely helpful for bias applications.
If you haven't used Paula's method for continuous strips you will need to join your strips to make one strip long enough for your project. Join your strips at opposite angles. The strips will form an "X" as you join them. The seam must be sewn at this angle in order to maintain the stretch of the bias.
I always use a minimum seam allowance, trim one layer of the seam allowance, and press the seam to one side, not open. When you begin to cover your cording, the end starting point will have a gap from the angles at the end of the bias. If your ends will be meeting and you want the look of continuous cording, be sure to start stitching 1 - 1.5 inches from the end. It is imperative to keep both layers of your bias from stretching or moving. They must stay lined up as you sew.
Your method of sewing will vary depending on your equipment. If you will be doing a lot of covering it is well worth the investment in a cording foot. You may also use a zipper foot. Until you have done a lot of corded projects it is best to do things one step at a time.
Line up the cording and covering. Stitch the covering on to the cording, sewing as close as possible to the coding.
Stitch the covered cording on to one layer of the project you are making.
Line up the last layer of your project and stitch into place.
Check to make sure all of your stitching is enclosed.
If you are doing a project similar to a pillow that will need a "unending" strip. I have seen two methods for ending the cording.
Overlapped ends: As you approach the end, pull both ends of the cording into the seam allowance, leaving a small portion overlapped to prevent any gaps.
Continuous cording As you approach the ends, trim the cord so that both ends will butt to each other. On the beginning bias turn under the raw edge. Use the beginning angle to cover the ending piece.
Seam allowances must be trimmed. All the layers of fabric tend to create a lot of bulk. A serged or zigzagged finish can be used on items similar to pillows. If you are using piping on something like a neckline, be sure to layer the seam allowance, to eliminate bulk.
Tips for Cording and Piping
Pat emailed me a great tip: "Sandwich cord inside fabric and place stitch witchery inside close to cord. Iron up to cord as close as possible (on outside of fabric). This will eliminate a row of stitching when applying to project. Hope you like this tid-bit." Thanks for sharing Pat!
Experiment on your scraps to get the "feel." If you are having difficulty getting the "feel" of the piping through all the layers, buy a yard of large cording at the upholstery section of your local fabric store and practice with it.
Trim a little girl's jumper with a contrasting piping at the arm holes, neckline, and pocket edges. Use a straight cut scrap to make a matching "scrunchie".
Add color and contrast to pillows by using piping or cording in the side seams.
Compliment your throw pillows with alternate trims. For example: If you make a denim pillow with bandanna trim, make bandanna pillow with denim trim.
Be creative with your scraps. Use them to make your own cording and "jazz" up a plain outfit by adding small piping at arm holes, neckline, and pocket edges.
Use piping in the seams of backpacks and pocketbooks to compliment an outfit. How to Cover Cording and Create Your Own Piping