Sashes are the simplest thing I can think about in terms of making, but even they can vary a fair bit when it comes to sewing details. Here are the few examples you can choose from. Each has its own merits and particularities.
1. One piece sash
It is as basic in construction as it gets. Fold the pattern piece lengthwise and stitch along the seam allowance leaving one end opened. Snip off he corner, but not too close to the stitching. By getting rid of unnecessary bulk the end of the sash will look neater when done. Turn right side out, tuck the edges of the open end in, then do a few hidden stitches by hand. Lay it flat and press into shape.
2. Pressed and top stitched sash
This sash is cut in one piece with a fold coinciding with the long axes. To make it, first press the seam allowances to the wrong side, then press along the center write side out. Sew together. To close the open ends – tuck them inside and top stitch if practical. Often when the layers are too thick you cannot do a neat job with sewing machine, few stitches by hand will solve the problem and hide the raw edges out of sight. For the very thin string-like sashes you can use a knot as a finishing detail.
3. Sash made of two pieces
It is not always possible to make a sash just by folding the fabric. The design might not be symmetrical or there is not enough fabric to cut it in one piece. Whatever the reason, the is a way around this problem. Cut your sash in two identical pieces and stitch all around , leaving a gap in any comfortable spot. I prefer close to the middle, for easy turning. Snip the extra fabric in the corners and turn right side out. Fold the raw edges of the opening inside and stitch it anyway you like. Press well6 paying particular attention to the corners.
4. Sash sewn into the side seams
This sash comes in two parts. Sew around the edge leaving the opening that will be inserted into side seam of a garment later. Turn right side out and press. Repeat with the second piece. 
