Your drawings probably look rubbish because you’re using an old biro that happened to be laying around. Nobody’s used one of these since cassette tapes stopped needing to be fixed, so throw that in the drawer full of batteries, twine and assorted electrical tape from whence it came.
Instead, go thicker. Go felt tip. Go a marker if that’s all you got. Because if your drawing skills aren’t very good, going thicker is always a good way to hide this fact.
Rough.js is a light weight (~8k), Canvas based library that lets you draw in a sketchy, hand-drawn-like, style. The library defines primitives to draw lines, curves, arcs, polygons, circles, and ellipses. It also supports drawing SVG paths.