Read a lot and write a lot but also edit your own work a lot, putting intervals between editing passes. There is no shortcut. A million words and you’ll be publishable. Three or for million and you might start to approach good. A third party editor might help but could also hurt and isn’t worth it unless the project requires it. Beyond what grammarly can do, labelled data is going to be in short supply— it’s mostly an unsupervised problem. Your most important editor will be you.