This tutorial is for beginner Photoshop users, so we will explain everything in detail; you’ll learn to use the vector drawing tools of Photoshop using our mouse… for now we won’t be using a digitizer tablet.
To start, the best thing to do is having a good sketch on paper; the size of the drawing doesn’t have to be very big because we will delete it at the end, but also it doesn’t have to be very small. Let’s scan it at 150-300 dots per inches (dpi from now on) to make sure that when we zoom we can clearly see the lines to work properly.
Let’s start with a white canvas (Ctrl + n), with a good size and resolution just in case we want to print in a big size.
To add new layers just click the button “Create layer”.
Let’s put the sketch in it (to do this, open up the file where you have your drawing, select everything with Ctrl + a and cut the selection with Ctrl + x. After that, paste it with Ctrl + v in the layer; if it’s small, alter it with Ctrl + t) and name it “sketch”, then create a new layer where we will put the trace of the figure and let’s name it “lines”. So we should have the 3 layers as it appears in the image.
Now we’ll use the Pen Tool 
to redraw the sketch; select it and activate the Paths Mode (left superior corner, the icon of the pen inside the square):
Now let’s click in the 3 sectors, and 3 little squares like these ones will be created:
If you right click (or click and maintain pressed) the Pen Tool, a menu like this one will appear:
Choose “Convert Point Tool”(the last one), press in any of the 3 squares we created a moment ago and drag it; this way you can twist the line and form what it’s called a Berzier Curve. Start shaping it in the way you want (we can always add or remove this squares with the Add Anchor Point or Delete Anchor Point as we want more or less complex shapes). You should get something like this:
There’s a little trick to create rounded shapes. It consist in create the vertexes and to add an intermediate point, after that you just have to move it with the Direct Selection Tool and that curve will be made by itself.
Make sure you are in the “lines” layer, select the Brush Tool 
and choose a hard brush (right click in the canvas, they are the first ones in the brushes list) of about 5 pixels with a different color from the one of the sketch, for example red:
With the Pen Tool selected, right click in the canvas and choose “Stroke Path”, and a window like this will appear:
Choose the Brush Tool and hit OK; this will generate a trace with the characteristics of the brush we chose before that will follow the previous route (this can be done with all the other tools).
If you mark the square Simulate Pressure we will obtain traces that will be smaller in the edges; with a little of imagination you can get all types of traces.
WELL DONE! You have you first trace, now do the rest 
The trick here is to make the traces in parts, in different layers so you can retouch comfortably superposed traces… like in this image:
It’d be ideal if you could make traces of the complete areas (for example we trace the glasses in one layer and the hair in another one, after that we delete the traces of the glasses covered with hair); it’s better to do it that way than starting tracing through the eyebrows and the hair with the same trace.
Now you have to work with the rest of the traces. When you’re done with it, we have to put together all the layers; we can do this selecting all the layers and pressing Ctrl + e. At the beginning it can be hard to manage with all the curves, but with some practice, the sketch can look as the original.
We have just finished our digital inking. You might probably notice that the lines are a bit boring as they aren’t modulated… patience, we’ll see that with more detail soon
Source
