This full illustration has been done on Procreate, and aside from a few tiny things here and there, you can follow the same steps to draw on Sketchbook too.
Step I
On opening procreate, you can view the Gallery where all your previously made artworks are displayed. At the top right corner click on the Plus sign to create a new canvas.
Step II
A submenu will open up from where you can choose canvas dimensions. We will create a canvas with dimensions specified on our own , so we will click this icon up here.
Step III
This will show, and now you have to specify the canvas width and height. For wattpad covers 512 x 800 pixels is the preferred dimension.
Step IV
Once the canvas opens, click on the Actions icon(in blue) and then go the tab 'Add' and choose Insert a Photo, to add the picture we'll be using as a reference for the vector.
Step V
The picture will be imported to your canvas. Now add a layer(by clicking the encircled plus icon), above that picture layer where we will do the outline.
Step VI
Using a thin pen brush(I prefer the 'Technical Brush' of the 'Inking' Set), draw an outline of the vector. You don't have to undergo the trouble of removing the background, because as we are going to draw the illustration of the picture on a separate layer anyway, you can later delete the picture layer altogether.
Here's how the outline turned out. (The visibility of the image layer is off):
Step VII
Once the outline's done, you can turn the layer visibility of the picture layer on and use the colour dropper tool to get the skin colour/cloth colour/hair colour etc of the reference picture. However, if you just wanted the vector structure as a reference and want to add your own skin tones and hair colours as per your preference which might not necessarily be the exact ones used in the picture, just go ahead.
Step VIII
Colour in, following the outline, keeping sure the layer you're colouring on, is not the same layer the outline is on, and preferably that layer should be below the outline layer. For filling in, I like to use the 'Syrup' brush from the Inking set. Mercury and Studio pen are equally good alternatives.
While colouring, make sure that while you are doing the outer boundaries of the vector, I mean, like, when you start filling in, you first retrace the outline with the colouring brush snd then use the fill tool right? So while you are doing that, one thing you might like to keep in mind is, setting the streamline and stabilisation feature of the brush to an approximately 50%-60%. This will help you to create smoother, straighter lines instead of jittery ones.
Once you finish, the result would look somewhat like this:
Step IX
Add in the details, step by step, first the skin then clothes then hair, or whatever order you feel comfortable with. Just be sure that the shading layer and base layer are different, and the different elements are on different layers, i.e, skin on one, hair on another etc.
While drawing the details, I like to keep the original photo as a reference beside. That's totally optional though, it just helps you to get an idea of which parts to shade, because once you have made the base vector it'll be difficult to continue to refer to the image layer, because you have to constantly turn the layer visibility on and off.
[ I'm not sure if this feature is there in Sketchbook]
Aaaaand the final result turned out like this:
Step X
Lastly add in the background colour, and add shadows.
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So, that was it for this one. I might make another chapter later going in details about the shading parts.
Hope this one helps!