In almost every tutorial, the bias binding is applied one side at a time. Here is an example, from oliver + s.
I prefer, whenever possible, to apply the binding in one pass. I just sandwich the fabric into the folded binding and sew. Store-bought bias binding usually has uneven sides. This is completely intentional. Lay your binding with the short side facing outward, i.e., on the outside of the garment.
I have a trick for mitering those corners. First, you need to mark the tip of the corner on the bias tape. Use a pin to mark the spot where the fold meets the edge of the corner. Be careful not to pull; binding is cut on the bias (bias binding!), which means it can stretch and warp. Be gentle laying it.
You can quickly test your template for accuracy.
Now, you have a template and a section of bias binding on which you have marked the apex of the corner. Finger press (just squish with your fingers) the binding so that is is open (not all the way, as you can see in the picture) and folded back onto itself at the apex of the corner.
Lay the corner of the template on the binding so that the point is on the fold. N.B.:The fold on pre-folded, store-bought bias is a little off center. Be sure to accurately position the template on the fold and not the center.
Transfer the template lines onto the bias. Sew on the transferred lines, trim, and apply Fray Check. Allow to dry.
Turn the trimmed bias binding, and refold it. Press lightly.