A tutorial of sorts... or as close as you will get to getting a decent one from me haha
I saved a jpeg of several stages of the painting I did of Livvy, nothing formal or strict, just when I remembered basically. There are really no special techniques to what I did, I painted with 100% opacity at all times, and a variety of brushes, but mainly a standard round brush (100% flow, 5% spacing, between 5% and 95% hardness, all sizes under the sun).
Layed out a rough sketch from a photo at my side (keep looking at the ref, never go too long without checking it over). You can be pretty rough with your sketch as any mistakes will be fixed in the painting anyway... the lines you put down are very temporary. And remember, i'm not attempting photo realism here, i'm trying to embellish what the photo contains and add my own input. You can play with the colours and all sorts of things during the process... the ref is mainly to just check that you are maintaining a likeness with regards to form and volume.
Start with a base midtone for the skin, then start adding in dark midtones to flesh out a basic shape. Go into it a little more with a dark tone to hit the real shadowy area's. Then you can start working up from the midtone to lighter colours, switching back and forth between highlights and shadows as you go. After you have a fairly structured base colouring for your skin, you can start to colour pick from your existing skin and blend area's in, pick out bits to emphasize, and really work on the focal area's, tightening the piece up as you go along (remember to add values of blue and green and red in places where you can see it, such as the eye sockets, cheeks, jawlines).
It looks pretty scary till you add in the eyes and mouth (I work quite segmented, and on a few layers, no more than 10 though). The eyes are the most important part of the image, the bit I struggle with the most, so its usually worth really drawing them out at the start so you can refer back to them often in your lines (some advice I should probably take myself haha
Hair... block it in, then, same procedure of dark, light, darker, lighter, highlights, shadows, back and forth all the time, adding in to the rest of the image as you go along as always. Balance is key, and working the whole image at the same time really helps with that.
Bg... I just paint in the original, and then play about with colours to along with the face.
Lots of tweaks and changes along the way, a few filters like sharpen and blur are used along the way, but its mostly straight forward.
I'm pretty crap at explaining myself, so if you have a specific question, just ask
PS. I spent 2 hours alone in post work, tweaking the eyes, changing the shape and size. Fixes are very easy in post, so try not to stress too much about that kind of stuff early on