A quick definition: Bias tape or bias binding is a narrow strip of fabric, cut on the bias (UK cross-grain). The strip's fibers, being at 45 degrees to the length of the strip, makes it stretchier as well as more fluid and more drapeable compared to a strip that is cut on the grain.
Before applying the bias, it is a good practise to prep it by stretching it under the steam (using your iron). It is trickier when the bias tape is pre-made (commercial) as you might have to fold and press it back to shape. In case of a pre-made tape, try pulling it lengthwise to stretch. The prepped bias will be narrower than unprepped version, but it will be ready to apply to the garment without the risk of it pulling the fabric or distorting the shape later on. It is absolutely vital to prep the bias for finishing curved edges, like armholes and necklines.
However in this tutorial we will be only looking at finishing the edges with 90 degrees corners, to easily achieve a neat mitered corner look.
Unfold you bias tape completely Pre-made bias tape will have one side narrower than the other. Unfold the tape and check which side is narrower. Align the right side of the tape facing the right side of the garment, narrow end of the bias tape to the seam allowance. Fold the short end of your tape down at a 90 degree angle to meet the raw edges and start sewing along the crease, do not forget to backstitch a little.
Sew all the way to the corner, stopping about a seam allowance width before the edge. Backstitch.
Take your garment out from under your presser foot, fold your bias tape up and to the right forming a 90 degree angle. Press neatly with your fingers
Align the bias tape onto the left along the raw edge and pin. Start sewing , leaving a seam allowance width from the edge, backstitch. Sew along the whole length of the tape required for your project.
Fold the bias tape over the seam allowance, make sure that folded tape is wide on the inside. Pin inside the seam and check on the inside, that the pin caught the edge of the bias tape. That is precisely why having one side wider is important. Tuck the corners in creating a lovely neat milted. Pin frequently.
Topstitch in a seam ditch, but always make sure the tape on the inside is overlapping the seam! That is why careful pinning is so important. Backstitch. P.S.: Some seamstresses like pinning along the bias tape, but with the satin tape, I found the pins leave the hole marks, so try to pin into the seam ditch, rather than the tape itself.
This technique is super easy to apply, even for beginners, just take your time and pin, pin, pin!