When it comes to learning to write by hand, one thing is absolutely key: lots of practice! Children with dyslexia will generally take a bit longer to learn to form letters and put them together to create complete words and sentences, so helping your child practice writing at home will allow them to achieve their potential more quickly.
There are a few things you can do if your child is surviving high school with dyslexia to make the learning process easier and more fun for your child:
Encourage good writing posture: Ensure your child is sitting comfortably at a suitably sized table and chair, and that they are holding their pen correctly in a tripod grip. Using a slanted desk may help with this.
Use specialist pen grips: Adding a specialist triangular grip to your child's pen or pencil can help make it easier for them to learn how to hold a pen using the tripod grip.
Use a tablet or phone: Allowing your child to form letters using an art app on a touchscreen device means they don't need to worry about how they're holding their pen, allowing them to focus all their attention on the shape of the letter and the sensation of writing it.
Let them write over letter formation templates: Children with dyslexia may struggle to remember how to form letters correctly by copying from an example. Providing them with a template to write over can help to familiarise them with the way it feels to write each letter.