Note: always slip the first stitch of each row, to ensure easier picking up of stitches.
For most sizes (34-40") 71st is enough, as there is plenty of ease, but for larger hips use 75.
Make the first triangle, center back, at the hem. Cast on 71 sts.
Row 1.
Row 2. K
Row 3 Using contrasting color, k1, k2tog, knit to center three stitches, sl 1, k2tog, psso, k to last three stitches, k2 tog, k1.
Row 4. K
Row 5, with main color, repeat rows three and four
Continue decreases at each side and center until five stitches remain, bind off.
Measure the long edge of this triangle and that will determine the width of the back times two. In other words, if it measures 11", then the back width will be 22".
Here's the triangle at the bottom center of the body.
The diamonds are made with 71 sts, and the decreases are done over the center three stitches as follows:
knit to center three stitches, sl 1, k2tog, psso,
Triangles # 3,7,10,11,15 and16, and the smaller triangles 13 and 18 have decreases (k2tog) one stitch in from the edge, on the right side only.
Since doing this pink version I have improved the neckline, and eliminated the triangles 21-25.
This is probably enough to get you started. When you have gotten this far, I will show you the rest, with instruction and pictures.