You can start from the chapter where the anime ended. Usually, there are online forums or manga communities that can give you the exact chapter number.
The Solo Leveling anime sticks closely to the manga. But there are instances where the anime might condense or expand certain events to create a more engaging viewing experience. The key plot points and character developments remain largely unchanged, though.
To start writing a solo leveling novel, you need to think about the leveling system. Will it be based on experience points, completing quests or something more unique? Also, plot out the early challenges the character will face. These challenges should be both difficult enough to be interesting but not so hard that the character can't overcome them at their starting level.
One way is by providing feedback. If you're a reader, you can share your thoughts on what you liked or didn't like about a light novel. This can help the author improve in future works. Another way is to promote the light novels you love. You can recommend them to your friends or on social media.