I usually start the drawing with placing a photo or some picture on the artboard. It's easier to start drawing if you have some kind of shape beneath.
There are some tools that are important to know how to use: with black arrow you can change the size of the lines and with white arrow you can control single anchor points. With pen tool you can make anchor points and lines and underneath the pen tool there's two tools with which you can either add or delete anchor points. Under Rectangle Tool there are other simple shapes too. Under Eraser Tool there are the Scissor with which you can cut lines between anchor points.
After placing the photo, I lock the layer. Then I take another layer so that I don't accidently move the photo while drawing. You can either draw with the pen tool or with the brush. I prefer the pen tool because it makes less vector points so it's easier to edit the picture afterwards.
I chose a bright pink colour for the line so that I can see it clearly. Later the line is removed so the colour doesn't really matter. I used the Ellipse tool to make the round classes and then made the rest of lines with the pen.
I used two layers: one for the classes and face and another for the scarf and hair. You can use more layers if it's easier for you. The bigger and more complicated the artwork, the more layers you should use because it keeps the parts that are already ready from accidently moving or changing. If there is lots of layers, they should be named so that you know what's where. You can hide the original photo by pressing the eye button so that you can see the artwork properly.
After finishing the outlines, I use Live Paint tool to colour the piece. You have to activate the whole artwork by making a square around the piece with the black arrow. Be sure to activate all the vector points. After you changed the artwork, you can no longer manipulate single lines with the black arrow, only make some chances with the white arrow.
The live paint tool can be found underneath the shape builder tool. As you made the artwork to Live Paint, all the layers joined. If you had some layer locked, the locked pieces of artwork didn't change to Live Paint.
Then I coloured the whole piece.
Finished! After colouring everything I removed the pink outline. I activated couple of lines from the scarf with the white arrow and turned the outline to red to make some wrinkles.
After that I saved the .ai version for the picture and made a .png out of it. I prefer .png over .jpeg because I think that the .jpeg changes the colours and with the .png you can make the background invisible. The final result can be found from the top of this tutorial.