My advice, when it comes to digital art, is to try and use a tablet or some sort of device that allows you to draw on the screen. Drawing on a device allows for more control. The time it takes for me to draw on my computer takes twice as long as when I am drawing in my sketch book.

It is important to take your time when doing digital art. Take time to make sure the  tones and the proportions are correct.

There are a few tips I can give when drawing digitally.

1. Put all the different elements of your art work on different layers.

For example: When drawing a person, put the eyes on a different layer from the rest of the body.

2. Do a light sketch first.

Plan everything you want in your drawing before you start. Keep the 'sketch' layer and remove the layer when you are finished. However, you need to delete the sketch layer if you are planning to do any smudging (For anyone using Photoshop for photo enhancement).

3. Do your 'smudging' on a separate layer from the rest of the art work if you are planning to do any. (For anyone using Photoshop for photo enhancement)

Although I recommend not to use the 'smudging' tool, there are times that you have to use it. Starting a new layer to 'smudge' your art work allows you to modify the area that you smudged without damaging the other layers.

4. Play around with the software before using it.

Look at the different effects, tools, and abilities the software can give you. Just playing around with drawing using the software will give you an idea of what you can use to make your artwork more realistic. (Play around with the types of brushes, erasers, effects, et cetera.)

Another  important aspect is choosing the type of brush that you want to use. Different types of brushes can give you different types of effects. Depending on the type of brush, you have the possibility to make things more realistic. 

The type of eraser you use when doing digital art is also important. (Photoshop has a specialized eraser that remove only the background depending on the tolerance percentage.)

I suggest trying to customize the brush and eraser you use to fit the situation.

Below is a video depicting the process of one of my art works. The total length, without any editing, would make the video three hours long. I also spent an additional three hours after I stopped recording; therefore, the total length for the art work is six hours.

https://youtu.be/xSY0YP_sV5k