My #1 recommendation for writing for a VN is to keep your motivation high. Losing motivation is the #1 VN killer.

You basically need to approach it with a "motivation maxxing" mindset. Write stuff that makes you happy. Try to not make the scope of the project too large so you lose your motivation later on. Also, whenever you feel inspired, take advantage of that and start writing. Don't wait and let the inspiration pass by. It can be a fleeting thing.

The other big piece of advice I have to offer is to treat your writing like sculpting clay. You're going to start with a big heap of wet, nasty clay when you start. But that's okay! Over time you'll take that hunk of clay and sculpt it into something beautiful.