TBH the only way to improve your handwriting is through PRACTICE, PRACTICE, PRACTICE.
But learning the basics, and knowing what to actually practice, is also an essential step. Enter the Spencerian Method: This super holistic theory book teaches you every minute detail of the writing process — from the angle at which you should sit and the individual strokes that go into every letter to how you should hold a pen. Once you get through all the workbooks, you'll pretty much be a pro.
Get a theory book and five workbooks for $23.84 here.