Redraw the outline in pen. Shade in the stripes and for the door snail, the texture.
Finally, colour in your shells. I used a pen to lightly create any grooves for the shell's texture. And that is all I had to do.
As I said earlier, slugs and snails can be the easiest subject to draw. As they are so slow, it will give you plenty of time to study them, but when you do, pay more attention to the shell. These are some of the most beautiful structures in nature and is worth a closer look and to practice drawing with. So get out there, find a shell and draw it as many times as you can. A perfect subject for beginners and professional artists alike.