I haven't been drawing much lately, but when I do draw digitally, I use a mouse because I can't get a graphics tablet yet.
I don't know how familiar you are with image editing software so bear in mind that some tips may require familiarity with them.
1) As with many things, practice plays a big part.
2) Some programs have a "stabiliser" or "smooth stroke" that smooths your lines so they are less wobbly.
3) If you're going for a "painted" style, don't worry about how "neat" it looks, especially when it's zoomed in. Trust me, it'll look just fine when you zoom out. Also, use a relatively large canvas.
4) If you want to do line art, use a vector graphics editor!! e.g. inkscape. You can easily edit the lines as opposed to a raster graphics editor e.g. photoshop.
5) Do a very, very rough sketch or outline on a separate layer beforehand.
6) If you're colouring in line art, fill in the area you want to colour with a single colour on a separate transparent layer and lock the transparency so you can't go over the lines.
It's probably still more time consuming than if you just use a graphics tablet and draw by hand (though idk because I can't compare) but it does make mouse-drawing a hell lot easier.
7) Play around with brush settings and brushes! Because you don't have pressure sesitivity and velocity is kinda hard to do, you need to make use of the other stuff.
I can't stress how important using a vector graphics editor is. It's surprising how much easier it is to do line art with it (using a mouse anyway). Additionally, don't spend too long on a small area. If you think it's good enough, leave it.Don't waste time trying to get it "perfect".