Gather your supplies.
(Reccomended) use lined paper.
Consider whether you really want to change your handwriting. Maybe you have a handwriting which is easy to read, or one that you can write really speedily with (good for exams, schoolwork).
Decide on a new font and write out the alphabet in both capital letters and lowercase letters: as shown in the picture. Do this a couple of times until you can write without thinking about it (much).
Compare your old handwriting and your new handwriting. Focus on the letters that look different. E.G: in the picture, the letter 'i' looks the same as the old one, so that letter doesn't need work on.
Practice writing a couple of sentences with your new handwriting.
Practice using your new handwriting in be everyday life, E.G: essays, notes, letters.
In no time, you'll be writing with your new handwriting without even thinking about it!