One way is to first break down the manga's plot into key events. Then, expand on the characters' thoughts and feelings, which might be shown more through visuals in the manga. For example, if a character in the manga has a sad expression, in the web novel you can write a detailed internal monologue about their sadness.
To adapt a manga to a web novel, you should analyze the manga's structure. Usually, a manga has panels and dialogue bubbles. Translate this into paragraphs and flowing text for the web novel. Pay attention to the world - building aspects as well. If the manga has a unique setting, describe it more vividly in the web novel so that readers can fully immerse themselves in the story.