The first step is to come up with an image, it can be a picture, a drawing or as simple sketch, the simpler the better. In this case I was made this very simple sketch based on an image from the internet.
Once inventor is opened, just import the image you want to turn into a sculpture, it will be helpful to have it as a guide for later tracing your splines.
Make a new 3D sketch, it is important that it is 3D. Locking into a view, in this case the top view, start tracing spline curves over the selected lines of the image. Using the sweep command you can later give volume to the spline curve. Once satisfied with the results, you can turn the view to a perpendicular one (right-left or front-back) and start pushing and pulling the spline points. NOTE: constantly check that the sweep function is still true after you move a couple of points around. Also, keep going back to the original view (top) to check that the image is intact.