You seem to be fairly proficient in measuring, gauging, and coming up with your own patterns. Perhaps your mom can help you with the measurements. Here's a simple baby sweater from the Lion brand site. I know you're doing an adult size, but this will give you an idea. I've converted this to knitting a baby sweater for my granddaughter and it turned out well. I like the simple rectangular shapes and the ease of construction in this. You would just need to measure the hips and length you want and divide this by two for front and back. be sure to give yourself a little extra for ease of movement, comfort, and the fact that you will be wearing a blouse under it. You have to make allowances in the pattern for that. You can do this! Welcome to the world of designing.