The reason that they wrote in this manner is that a quill pen and free-flowing ink are very hostile to abrupt changes in the direction of the pen. If you think of it as painting a fine line on the paper rather than writing, you will get the idea. There is no substitute for practice. When you starting writing in school, it was tough at first, your hand got tired, and the letters never seemed to look right. After a great deal of practice, you got better.
Writing is a skill.
Any skill improves with practice.