You're probably going to have to go the old fashioned route with pencil, paper, a calculator and math skills. It shouldn't be too bad if it's a simple shape. My son is an artist. He reproduces cartoons or designs to paint on his jacket. He splits the entire picture up into tiny grids on graph paper and copies it in small areas at a time until he's got the entire thing done. He starts by charting the top, bottom, and sides of the design first (the main shape), then adds the details.
You can test your designs by making them small at first and knitting potholders with fair isle or intarsia. You would then have to figure out via math how to enlarge them onto something like a sweater and have it centered correctly. You would center it by the widest width of the pattern and the tallest height of it.
You would use the gauge listed for the pattern to figure out the exact placement. Look at the row gauge and figure out how many rows per inch. Look at the pattern to determine how many rows to knit between the ribbing and neckline. Divide this by the number of rows per inch. This should give you inches.
Example: My pattern tells me to knit 12 rows. My row gauge says that I should have 5 rows per inch. That's 2 1/2 inches of knitting. Half of this would be 1 1/4. This is my vertical center point. My stitch gauge says that I'm supposed to have 5 stitches per inch. I have 20 stitches total across. That's 4 inches total across. My horizontal center point would be 2 inches across. I would mark this center point where the vertical and horizontal lines cross. The center point of my design would go here. I then figure out by measuring from the center point which row the bottom and side edges of my design would start on. It would be easier to sketch this placement all out on graph paper, one square equaling one square inch. Hope this makes sense to you.
This should be easy to do on graph paper if you've ever done counted cross stitch. Find the center point. Count the number of stitches to the left and right of center point. This is how many inches across you want the design to be. Graph the lines above and below this center line.