During high school, my term exams (quizzes) required handwritten answers spanning multiple pages.
My handwriting was barely readable.
Based on some observations I started to write the letters with,
1) consistent shapes
2) same size
3) equal spaces between the letters and words.
This made my handwriting very readable but not necessarily pretty.
A few years back when I took an interest in typography I realized why this technique worked.
A printed page mostly uses a single font (letters of the same “shape”), same font size (same size) and consistent spacing between the characters (equal spaces). This among other things makes the page very readable.
I hope this technique helps someone improve their handwriting skills.