I am not the master in creating perfect Illustrations using Brush or Pen Tool but still I am trying to made this tutorial to help the less experienced or beginners. In this tutorial I will draw this pretty girl ;-) wearing a carnival mask.

Step 1:

For the reference you can create your own design or you can use any downloaded image. Personally I do prefer draw outline or sketch first. Just draw the outline for the base what you want.

Step 2: Lock the layer of base image or Reference image & start with a new layer using existing base image as a guide.

Step 3: Lets start with the line art. You can start with tracing the outline and then continue with the inner details. I did this by using the Pen tool (P). (In case of using brush tool you should go to Object Path > Clean Up, to fix the empty paths that may cause you unnecessary problems.)

Step 4: When you finish your line art select the object with Selection Tool (V) and start adding color using the Paint Bucket tool (K) or Gradient Tool (G).

Step 5: After you add the proper fill colors & gradients dont forget to press the Expand button. (Expanding objects may be helpful when you want to use an object that is native to Illustrator , such as a mesh object in a different application that doesnt recognize the object.)

Step 6: Weve finished adding colors but it looks like we dont have enough depth in the design.

Step 7: In order to give more depth, we need to add the shadows and lighter areas to the Illustration. So you need to select the desired shape with the Direct Selection Tool (A). Draw a filled path with your color around each area that has the same color.

Feel free to experiment. This is how my Carnival Girl looks at the end :-)