I think that I would suggest maybe looking up YouTube videos on it. There's some great artists there. I just started digital art 6 months ago and I'm still learning but I'm far better now than what I was before! Try to set aside time each day or week to work on it. You'll be great!
- Don't use black to shade unless it's a black and white photo, black is for artists who don't know how to use color - layers layers layers, layers are your best friend. Use them to organize, use them for shading/lighting modes, use them - Use stabilizers for tricky line art, in Paintool Sai this is the top right option on the task bar - Try not to overdo it with the blur tool, I see this mistake a lot with beginner artists - Don't use super bright or saturated colors, I see this mistake a lot with beginners too specifically with digital art - Experiment! There's a lot to discover with digital art and it's no risk at all to try something new!
One think I noticed after losing a lot of art, I thought I was better than I actually was. You don't really lose the skill, because it is not actually the skill you lose, it is your quality standards. If it is not complementing your skills, then you need to relive painting or drawing to see what you have to work with. A lot of things change over time, like a long time ago you knew less about how somethings look, and now that you know more, you just need the skills to do what you know would look right.
To get back into art, don't try to do anything from your imagination, because you don't improve, you just keep what you knew and only able to use that. Learn to become better with references. Then it is just your skills improving to be able to do more. Your wisdom will not improve until you know the details of what something looks like. If you know exactly what something looks like, then do that, and see if you were right. While you do, you might learn what you didn't get right, and you will do better next time. This is how you study subjects. You learn as much as you can, then draw what you know to see what you get right, so you can work on everything you did wrong. Then you do it all over again until you know everything about how something looks. This can take a long time if it is anatomy while animal anatomy might be easier, because more hair, less details. You just learn the hair style and directions.
Hair is like a material, and with a pencil it is easy, you just draw the hair strands, but with digital art, if you paint the hair strands in the same color it become a flat color and not show any brush strokes. You need to keep changing color variations to keep showing the strokes and not over do it. Areas where you need darker colors you select darker shades while still doing the color variations, and where it needs to be lighter, select lighter colors. Learn from images or real life to get to know how to create the same look as the references, and after you get good at it, you can do it without a reference.
One thing you always need is a filled in solid shape. If you draw hair on an animal while you have a black background then the background is removed and you see through all the hair, and see things behind it. I started to add a black section behind the layer with the hair, but it can be better to change how you sketch. In traditional art, you sketch with a pencil, because you are just getting a visual idea of how things could look, and after you see it, it need adjustments to make it look right. The reason you use the pencil and not a large roller to fill in the shape, is because you can erase the pencil line to adjust the shape. In Digital art you can erase everything, so you can use anything you want. You can fill in a shape and trim it or add to make the shape look right, Then when you paint everything inside and the shape needs to be adjusted, you can go to that layer and make changes.
A lot of useful things to remember is because this is digital, and it is not traditional, where your mistakes are permanent. While you can resize things, morph the shape, move things around, add and remove anything, paint a full body and put it behind something else and show part of it. You can hide things and show them, so they are not in the way, and there is a lot more that each program can do, like put a flat wall of bricks into perspectives for a side on a perspective block for a building. With some program having nice tools like Liquify that can stretch, enlarge or move things around, and Stroke that can draw a line around any shape to help make most of the line art, you can try free programs and find what they do that can be useful and use it when needed.
If you want to do cartoons Stroke can help, even if you want to add line weights to the lines. I work with a mouse, so I am not depending on pressure sensitivity. If the line get wider I draw add another line to make it wider and fill in if needed.
See what you can do now and improve on it.
I started a drawing and in the next 2 years, I lost all of my art, I installed a program that let me see files put on a dvd while using the program to use the dvd as a hard drive. Then I found one of my unfinished artworks. I didn't really draw much after that, because I wanted to study how different materials looked, and the first thing I learned was you will not see anything in the dark, and in the light, the materials react to the light. Some grains looked deeper and some were just darker. There is a gloss level, and things like refraction, reflections, glow, bounce color or more exact glow when the light has the same color in it or look black with it has no color that is in the light. I use to think the light hits a surface, and that surface is lighter, now I understand without much light, everything is darker, and where the light is brighter, the full colors show their true colors. In a reflection you see the light source. A whole area is not brighter unless it is reflecting a light. Reflections in colors reflects just the light, and not any dark. Metal can reflect both. Because I learned so much from life in the two year without drawing, I knew how the skin needed to look. Before I wanted to avoid lighting, because I was not good at it, and everything looks dark and muddy. When I repainted it to add lighting I included everything, and then I saw I became the artist I wanted to be.
What are shapes, but a structure to put materials on, and together, you can make anything look real. 3D programs do the same thing, so it is better to change how we think about digital art, to become better at it and become faster. Use what you have to work with. Create a list of what to do to make creating the art from an idea to a finished product. Like a list might start with a sketch. You can draw lines or fill in a shape. sketching out lines can take longer because you are trying to follow the border, while filling in just goes through the middle. It is a shorter path, and you need a filled in shape anyway. If the filled in shape is black, you can add a color layer on top to paint the surface colors and use the right size soft brush to get the right size curve, so your list will be sketch a layout of the idea, improve it, and then paint in the shapes for a more finished look. You could include not start with a huge canvas, because the brushes are too small. You need the brushes to paint any thing, like in one stroke paint a blue for a sky dome with a large soft brush. Become better at working with the pixels to improve the quality of your art, or focus on bigger problems than they fine details. Rules you create from what you experience that can help, or fill you with regrets. It is the do's and don'ts from what helps you create art quicker and easier. It is becoming more efficient, and it makes these things important to remember. The list also needs an order, because you can't start drawing a sketch after you already started painting everything without going through the process, and when you make that mistake you don't know what you will do next, or why something don't look right. With a sketch first, you plan everything out, and after that you follow though to finish each thing. You know what you will do and what you want to do next until everything is finished. You feel better and even improved things to look better along the way, and you finish and never give any part of it another thought or spend hours looking at what could be wrong.
As already mentioned it's hard to give a general advise to someone when you have no idea about what they need improvement on. Just get started is my best advice to you. Experiment a bit and see which direction it takes you. After a while you can ask for some feedback on a finished piece perhaps.