I was fortunate to be taught Marion Richardson [0] style handwriting aged 8 or so, and decades years later my handwriting is still universally complimented. I hand write letters for pleasure and take notes by hand for more profound impact.
If you look after a young person or are willing to go relearn it yourself I highly recommend the discipline and style that Marion Richardson demands. While learning it at first it looks childish, but it provides a foundation that you will immediately adapt with your own flavour. It got dropped in UK schools in the 1980s for being too old fashioned, for which I read 'difficult'.
Sadly I cannot recommend any modern books or worksheets (through my own contemporary ignorance) but her original books from 1935 still stand up [1]
[0] https://press.uchicago.edu/ucp/books/book/distributed/M/bo13...
[1] https://collections.vam.ac.uk/item/O1326657/writing-and-writ...