Hello lovely anon! First, I’d suggest for you to read my rules, I tend not to answer request that don’t start with ‘Hi’ and end with ‘Please’. Or at least have these thrown somewhere in here.
Anyways. I won’t write guides on subjects I’ve never lived myself, first or second hand, and I’m not disabled. But. I asked my little google friend and typed ‘Write a disabled character’, and I found these little links that could help you.
It’s simple. Do your research and I’m sure you won’t need any how to. Doing your own research and reading things from different point of views is actually better than a ‘simple’ how to, which are often inaccurate.