Using a pencil, begin with very simple shapes. The goal is to create a framework and then build on top of that, slowly adding in more shapes and details. Start your drawing by creating a stick figure type of pose, using circles to indicate Ahsoka's head, shoulders, hands, waist, and knees, and then add lines to connect them, as shown in the example. Remember, it's best to keep your lines very light. This way you can easily erase and redraw them if need be.
At this point, before we get too far with the over all pose, it's a good idea to draw the outline of Ahsoka's head tails. This was can make sure everything looks right in terms of size. You don't want Ahsoka's head to be too small in comparison to her body. She's still a teenager, and proportions can help reflect a character's age. Once you're happy with the head tail outline, go in and add the little triangle shapes around her face, as well as ovals indicating where her eyes will go. They should be about halfway between the top triangle and her chin. Let's also take this moment to draw a few simple lines and shapes to indicate where her belts and accessories will go.
At this point it's a good idea to start creating the outline of her body. No need to make anything too detailed right now -- just quick, simple shapes and outlines to show you what her arms look like, legs, where the top of her boots are, etc. Again, make sure your pencil lines are very light -- you don't want to make anything too final yet; you may still want to make a few changes to your sketch.
Now you can start placing your smaller details, such as the stripes on her head tails and the diamond shapes on her leggings. This is also the perfect time to draw in a rough circular shape to indicate where her nose will be, and a little squiggly line for her mouth. Obviously these will be fleshed out a bit more in the following steps, but starting with simple, loose lines like this can really give you a nice feel for where everything should be. You don't want to draw a perfectly detailed mouth, for example, and then realize you drew it a little too high on her face. It's easier to start simple, figure out the placement of everything, and then go in and tighten up your detailed lines.
Just like the last step, we're adding in a few more details, such as the markings on her face, the edges of her outfit, gloves, etc. You can also flesh out the shape of her belts a bit more, too, including her belt buckle and the little metal squares on either side of the buckle that hold her lightsabers when she's not using them. Last but not least, go ahead and draw in the basic shapes of her lightsaber handles. Ahsoka has her own unique way of holding the sabers, so the pose I've provided reflects that.
If you want to give your drawing a clean, finished look, you can add inks over your pencil lines. This can be done with a variety of pens or markers (personally I'm a big fan of Faber Castell's artist pens). Just trace over the lines you want to keep, and when you're done (make sure you let your ink lines dry), go ahead and erase the pencil lines.
Now, using bolder pencil lines, you can use your lighter sketch lines as a guide, and essentially trace what you've created thus far. Having basic shapes and a rough sketch will help give you the confidence and skill to create a great looking final drawing. You can add in as many or as few details as you like, and if you want, you can add some shading, too.
And here's a color version that I created digitally in Photoshop. You can use markers or crayons or whatever you like. However you decide to finish your drawing is completely up to you!