Another way to start is by looking at different fanfiction tropes. Maybe you want to do a 'what - if' scenario. For instance, what if the Loud family won the lottery? How would their lives change? You can build your story around that concept. Also, it's a good idea to read other Loud House fanfictions to get inspiration and see how other writers handle the characters and plots.
First, think about the characters you want to focus on. For example, if you love Lola, you could write a story centered around her beauty pageant dreams. Then, come up with a unique plot. It could be something like Lola getting lost on her way to a big pageant and having to rely on her siblings to get there in time.
You should also study the personalities of the characters in the original show. If you want to write about Lincoln, remember that he's often the mediator in the family. You can start by creating a situation where his mediation skills are put to the test, like when there's a big fight between the sisters over a shared room. Then, just start writing and let your ideas flow. Don't worry too much about grammar at first.