1. Make it legible.
A legible handwriting brings clarity in your thinking. Remember, handwriting is a way of communication. Communication has to be clear. If you are unclear in communication, it is because you cannot convey your message or you are not sure about what you want to write. Either ways there is a confusion in personality. Legible writing is the first way of personality amity development through changing handwriting.
If your m's look like w's or n's look like u then there is no legibility. If you forget to cross the t's they will look like l's. If you write closed e and forget to dot the i then they will look the same. If you just thread some letters at the end then the writing is illegible.
2. Make it consistent.
The writing should be consistent. However it is, it should be consistent. That makes you consistent in behavior. A person whose behavior is consistent is more reliable than a person who is less consistent in his behavior.
If the slant keeps changing from left to right;if the size keeps varying from small to large; if the pressure keeps changing from heavy to light; if the writing significantly fluctuates frequently with moods and stress etc then there is a lack of consistency in the writing and thus in the personality.
3. Learn Graphology
Graphology is the science of understanding personality from Handwriting and signature. You can learn Graphology and use it to analyse your self. Then you may decide what traits you wish to change in your personality and work on it by changing the writing as well as the habits.
It is easier said than done but changing Handwriting certainly changes personality.