Make sure you always add some sort of outline for hard-to-see things. Also, make sure you are adding highlights around which are close to the main color, but not too close. Darker shades and lighter shades should almost always be around the edges, and make sure you have a variety of at 2-4 shades surrounding the character. The background should always be simplistic, but still have enough shading and highlighting for it to blend in with the main scene, like in my Among us memes. Also, for beginners, you should always take a transparent background character and try to replicate by drawing side by side. This improves both accuracy and your feel with the mouse / mousepad. Normally, you should ALWAYS use small thickness for highlights and shades. This makes it more realistic.
OK, now WHERE to shade and highlight. Usually there is always a place of light located. Take a light bulb for example. image that 0 is the light bulb, and 1 is your character. Lets take an example here
now if you look at the light bulb, it should be shining all around it, meaning your character must be immersed to light. Wherever your character is having light shone directly at it (in this case, the top left of 1), should have most highlights and little to no shades. Now, where do the shading go? exactly the opposite. Most shades at the spot where light barely / doesn't go to, which would be the bottom right of 1. Now lets talk about 3D shading. Lets say your character's glass piece in among us pops out a bit. That means that most of the time, there should be shading opposite to the light and prior with highlight (which is where light is). After this, make sure that your shading and highlighting is similar with the base coat, but not too similar in which it needs some sort of outline. In order to show 3D hands, make sure you first study hand anatomy. Look at your hand. Do what the character is doing and look at it from the drawing's prespective. Try to look at the shades, the pointiness, the bumps, everything. Then apply most of it to the drawing. Make sure these drawings are not rushed, since my drawings actually take like 30-40 minutes per frame. Everything should be perfect.
Finally, the texture. Make sure your objects arent too round, or else it may mess with the flow. Textures or material should usually be drawn with a large sum of shading to blend it in and not make it pop out too much.