Well some tips would be not drwawing on your phone
but other than that start off by making light marks and the deeper the object gets as it goes.
The darker the shade needs to be also If you are going to shade you probably also need lighting, well atleast in the instance that you are drawing people.

I have illustrator but have yet to buy a mouse which is why im using stylus and phone atm

new wip

Really enjoy the newest creation, would love cnc

Since i've been gone: moved to my pc and ought a mouse, quality of images has gone up

the last one is awesome.
some tips:
- use pathfinder to crop lines and shapes inside other shapes by duplication and intersect.

- take a good look at width tool, you can make it more stylish and acceptable changing line width.

- dont rush your pen tool, always keep everything fluid, a tip is to use the anchor tool to smooth lines.

- make forms without contour to make shadows and light, you can use gradients on those for style.

the rest looks fine, I recommend you to experimenting with mesh tool to make gradients with more control, in some cases it is necesary.

uh made this

your art lacks depth, try to feel the form before you vector something.
your shadows can be done better, test with line direction before applying shadows, some of your shadows are cut.