The layer selection too is a lifesaver when it comes to shading and it saves a lot of time and makes it look better overall!

First go to layers and select this very top one called selection layer

Use the bucket tool to fill in any space you do not want to shade, then press the little button with the square and circle sorta thing

Now add a new layer (I reccomend putting it above the lineart and coloring as it makes the shading look better) and use this guy at the bottom to adjust to opacity (I perfer 50-60 opt)

Now you can color the area you want to be shades and the color will stay within the line! (I didn't find out about this for a loooong time)

Then you can use a blur tool to polish up the shading and make it look more natural

Once you've finished shading, go back to the very top and press the box with the line through it and the blue highlight should dissapear

And bam! Your shading is done! Hope I could help yall

(Another tip, if you have a background, I'd recommend hiding it then using the layer select the you can unhide it. It saves a lot of trying to fill in individual parts of your background)