One great OC Blue Beetle fanfiction could be 'The New Blue Beetle's Adventure'. It might explore how an original character interacts with the Blue Beetle's world, like facing off against the same villains and making new allies. Another could be 'OC in Blue Beetle's Team' which focuses on an OC joining the Blue Beetle's fight for justice in a unique way.
First, come up with a unique OC. Think about their personality, abilities, and backstory. For example, if your OC is a martial artist, how did they learn their skills? Then, decide on a plot. It could be something like your OC helping the Blue Beetle fight a new villain. Also, study the existing Blue Beetle lore so your story fits well within the universe.
One common theme is the OC being a sidekick or partner to Blue Beetle in his superhero adventures. Another is a romantic relationship developing between the OC and Blue Beetle.
Another idea is a time - travel fanfiction. Blue Beetle and Black Beetle could be sent back in time to a crucial moment in their predecessors' lives. They then have to make sure not to disrupt the timeline while also trying to gain some knowledge or power that can help them in their present - day battles. This would allow for exploration of their origins and how they've evolved over time.
You can start by looking on fanfiction websites like Archive of Our Own (AO3) or FanFiction.net. AO3 has a wide range of fanfiction, and you can use the search function to specifically look for Blue Beetle x OC stories. FanFiction.net also has a large collection, but the quality can vary more. Another option is to check out Tumblr, as some users post or reblog fanfiction there.