Holy cow, I just wrote you an extensive reply to your post and it went into the twilight zone. Here we go again!
I have viewed both the drawings you want to be able to create and also the drawings (cartoons) you have already created. Always remember that cartoons are important creations. I have a few suggestions: First get yourself a 6x6 spiral sketch book, a 2B pencil, (I use a mechanical pencil) a .5mm lead pencil, and a carpenter's pencil that you'll need to sharpen with a pocket knife. Include a soft, white small eraser. Begin sketching both cartoon figures and then include a small still life-- oranges, apples maybe, coffee cup, etc. Do this over and over again with new subjects. Begin to sketch lightly. Next FILL IN SHADOWS. None of these sketches are to be painted. You may wish to indicate light source direction and/or make written notes. Create these fast as you can. Later, you can return to the original sketch and recreate it with more detail. Carry your sketch book and pencils with you at all times. My guess is that it will become a new best friend. My other guess is that you already have the abilities you need, you just need to sharpen your skill set more. At some later date, you may wish to return to your sketch book as a sort of fertile ground
for painted characters. My last guess is that after you redraw your selected character (with plans to paint it), you'll also wish to include painted shadow detail. At that time, you will have achieved the creation you wanted in the first place.