One aid you might want to consider picking up is a French Curve. I have never used on in my designs, but I probably should. A compass and circle templates can also be helpful. Keep in mind though, most of your designs will have to transfer to a curved surface, and may not look right once you do this, so what might look good on paper might not look as good once transferred. I work backwards these days... I draw it on the stock first, and if it looks good, then I photograph it, then I carve it and photograph it afterwards.
If you want to try something, take one of those pieces of scrap you sawed off of you blank (assuming you are working with a stock blank), and glue it to a piece of 2x4 and shape it roughly to the curve of where you wish to carve. Sand it down to a finish that you would have on your rifle before starting carving. Draw you pattern on there... It doesn't matter the pattern, but just one you are hoping to execute. Use your eraser frequently, and get it as good as you can. Then carve it to totality. It will really improve you abilities in all aspects, and having done your pattern once, it is likely you can see where you made mistakes, and correct for them on the real thing. If necessary, repeat this until you are comfortable carving your stock.
M. E. Pering