1. Our starting image is the one below. As you can see, it's a little bit grainy.
2. After some coloring adjustment, you'll get this.
This adjustment is not necessary but it will make the image prettier. You can get the coloring effect by following this icon coloring tutorial in Photoshop. Remember to flatten your image after you get the coloring you want. In PC you can use this shortcut CTRL+SHIFT+E or go to Layer > Merge Visible.
3. Now, duplicate the image 4 times. You'll have the original layer, layer copy 1, 2, 3 and 4. Make layer copy 3 and 4 invisible by clicking the eye beside each layer in the layer palette (usually located at the bottom right).
4. Leave the original layer and layer copy 1 alone. We're gonna work with layer copy 2. Apply cutout filter. Go to Filter > Artistic > Cutout. Use the setting below:
Number of Levels: 8
Edge Simplicity: 4 or 5 or until you get something similar to the picture below
Edge Fidelity: 1
5. Set this layer copy 2 to Multiply.
6. Next, we're working with layer copy 3. Click the box where the eye icon used to be, to turn the layer back to visible mode. Go to Filter > Artistic > Dry Brush. Use the settings below:
Brush size: 10
Brush detail: 10
7. Set this layer copy 3 to Screen
8. Now, make layer copy 4 visible. Go to Filter > Noise > Median. Set the radius between 5 to 7. Mine is set to 5. Then, set the layer mode to Soft Light.
9. Okay, now we're done with all the layers but there's one final step that will determine the outcome of this cartoon effect on your picture. Adjust the opacity of layer copy 2, 3 and 4 to get your desired comic-like effect. For instance, you might want to lower the opacity of layer copy 2 and 3. These are the opacity settings I use for my image:
Layer copy 2: 45%
Layer copy 3: 41%
Layer copy 4: 100%
And you're done! Here's the result: