1. The Sketch 
1. The Sketch
For the sketch, try not to dwell on it for more than 10 minutes or so. If it's really not working, don't be afraid to start completely over. A great thing you can do it select an entire area with the select tool, and just rescale/resize it. See if it works a little taller, wider, etc. This is the perfect time to try a new pose, a new angle, etc. If you take too long on this step, I feel like you could lose some of the fluid movement feeling that could end up working well in your finished piece.
For this project, I took some inspiration from Cherubiims site, and found some real carousel horse pics too. I wanted to create a soft-beautiful-dreamy unicorn/pegasus for my splash page to replace the love-nikki carousel that was there before that I really didn't have much attachment to except that it was pretty and lolita-esque. Don't be afraid to use reference photos, and then still change up the poses! See if you can capture the feeling from a reference image rather than capturing it structurally or photo perfect, for example.
2. Line Art
I wanted to go for a pixel look on this, so I stuck with a "pencil" tool that lets you draw pixel by pixel. You could of course do line art in Sai, which helps you make smoother lines, or whatever you prefer. I wanted to make sure the lines felt dynamic, because doing line art is often when your piece can become "flat" feeling. It loses all the energy from the sketch phase. What I did here was leave some of the lines a little more messy than I typically do, and I made sure to vary the line width in certain areas to keep it interesting.
I tried not to get too caught up in adjusting every little pixel, but it was SUPER hard because I'm used to working on very tiny pixel scales! I tried to adjust the line art primarily based on how it looked more zoomed out.
3. Coloring 
I like to just totally fill a layer (like hair or body) with a solid color and keep it on a separate layer from the line art. Since this piece was done in a pixel style, it was very easy to select the area I wanted to color, make a new layer, then fill in or paste that color in the new layer for editing.
4. Adding Shadows
The easiest way to do this I think is to "preserve opacity" on a layer. Many programs have this feature, you might have to look up for your specific one. Basically, no matter where you color on the layer, it will only color on an already existing color. It helps you not have to clean up as much outside of your linework later on.
Remember to consider your light source, and think about the shape that you're adding shadow to. Also, I like to add a "darkest" shadow to certain areas for some more visual interest.
5. Adding Highlights
This is like the "icing on the cake" stage for me. Add shine to the skin, color bounce lights, hair shine, etc. The highlights and little extras you add can really make the piece your own. I have learned that the more tiny details you add, the less it actually adds to the overall piece. If I find myself working too small and zoomed in, I zoom myself out and look at the piece as a whole. Is the tiny addition I'm focusing so hard on really adding anything that most people will see, or no? If no, I try to leave it and add larger highlights or additions that will add to the piece in a more meaningful way.
6. Finishing Touches
From here, you can play around with things like coloring the line art (A favorite finishing touch of mine to add a little more depth to a piece). I'm not great with backgrounds, but of course you could continue on and make a background element at this point. You could play with filters on each color layer and see how they affect the piece overall. You could give the whole piece a wash or fade of color to unify it, or mess with other image effects in photoshop and so on. I also like to add animation on the final stage, even if it's a simple bob up and down! I hope this helps you with your own art process ^_^
