To write a unique 'the loud house it crossover fanfiction', think about the settings. Combine the familiar locations of Royal Woods from 'The Loud House' with the eerie places in 'IT' like the sewers or Neibolt House. Also, consider the different powers or skills the characters have. The Losers Club's knowledge of Pennywise could be shared with the Loud siblings, and vice versa, like the Louds' creativity in getting out of tricky situations could help the Losers Club.
One way to create a unique 'the loud house it crossover fanfiction' is to explore new relationships. Have a character from 'The Loud House' form a special bond with a member of the Losers Club. For instance, Leni could befriend Beverly and they could learn from each other's fashion sense while also fighting Pennywise. Another aspect is to introduce a new plot device. Maybe there's a magical object in Royal Woods that can defeat Pennywise, but only if the characters from both universes work together to find and use it.
First, focus on the characters' personalities. For example, make use of the Loud siblings' diverse traits. Then, blend the horror elements of 'IT' with the family - centric theme of 'The Loud House'. Maybe have the kids from 'The Loud House' use their family bonds to overcome Pennywise's fear - inducing tactics.