It depends a lot on the age of the pupil. You can try Write From The Start, but without an adult you might find what they’re doing isn’t what’s required and bad habits might get more entrenched.
Depending on the age of the child general pencil control activities eg dot to dot can help as can tracing over handwriting patterns to develop the correct movement.
Is the poor handwriting due to developmental coordination delay (dyspraxia rather than dyslexia, dysgraphia)? If so more general fine motor activities like flicking marbles, screwing up paper, pegboard pictures, putting clothes pegs onto and off the opposite shoulder etc. Will help build strength & coordination. You LA / PCT will likely have a preferred programme.