Read your own work aloud. Seriously, it works. Go slow and take your time reading every single word you’ve written. Even if it makes you cringe, even if you hate the sound of your own voice, even if you hate the words.
Reading your work aloud will help fix any grammar mistakes and help you edit, you will be able to see where you want your pauses (commas) and get a better sense of how well your dialogue really flows.
If you stumble over a sentence because it sounds choppy when you read it you can change the tense or use different nouns or adjectives to make it better. If you feel like you’re being too blunt you can add things to make your sentence more lyrical or descriptive.
Even if you don’t like to do it or don’t think you need to do it, do it. Read your work aloud.