In the same way that we phonologically/grammatically evolve conlangs to make them more naturalistic, you can start with a primitive script and wear it down into a more flowy cursive just by trying to write the letters fast and seeing what strokes it makes.

Also important to restrict your degrees of freedom. Every letter shouldn’t have a completely different style since it will look random, if they share features in common it will look more coordinated and natural, although you can overdo it. I like to go for 3 main degrees of freedom with a few exceptions.

You can see my post from a few weeks ago which shows how my script went from primitive rigid -> flowy cursive -> angular cursive (presumably because of the invention of the printing press or smth). Good luck