If you are drawing an outline that you want to convert to CAD using raster to vector conversion, it needs to be drawn as a single, strong, unambiguous line.
Below left is a typical hand-drawn sketch. The sketch is not suitable for automatic raster to vector conversion for two reasons.
First, there are ambiguous lines where the artist has re-sketched parts of the drawing several times. An example is at the top of the left hand leaf (arrowed in red). Which line should the raster to vector converter follow?
Second, the sketch has been drawn in soft pencil using different pressures. This means that although parts of the sketch will come out black in the scan, others will be faint and broken (arrowed in blue).
Above right is the same sketch with uniformly strong and unambiguous lines. This is what it needs to look like if it is to be converted to CAD using automatic raster to vector conversion.