Clark is obsessed with Batman.
Older Superman who sees this younger superhero running around Gotham. Inexperienced in vigilantism, desperate and naive, in need of support. The first time he saves the man, Batman glares at him, and tells him to stay out of Gotham.
Does Clark listen? Of course not. It becomes his nightly routine to tune into Gotham, keeping an ear out for crime. Trying to guess if Batman will stop it, or if the criminals will get away. He even conceals his identity and explores Gotham, hoping to be caught up in a crime that requires Batman.
After a year, Superman approaches Batman to invite him to the Justice League. He's rejected immediately. But if there's one thing journalism has taught him, it's that you have to appeal to people's weak spot, persuade them to give up their reluctance to satisfy their morals. All it took was one intergalactic conflict that affected Earth, including Gotham. With no choice but to protect his city, Batman joins force with the League. If Clark might've instigated the conflict, well, that's no one's business but his.
After that, Batman admits having the Justice League would be nothing short of an advantage. But he doesn't exactly know how to approach them. They're all older, experienced heroes, he's still a relatively new vigilante. He can't expect them to just accept him. Except, there's a certain Superman willing to help.
Maybe Clark creates situations to make Batman dependent on him. Maybe he starts a rumor. Maybe Batman's history get revealed. Maybe Bruce's identity is at risk. Maybe Batman's plans are being sabotaged. Maybe Batman needs to be voted into thee team, or needs his plan to be taken seriously. Clark uses his seniority and founding member status to help integrate Bruce. Maybe other heroes who aren't allowed into the League as easily hate Batman for pulling strings (strings he didn't even know existed). So many situations of Clark's own making that have Bruce turning to him for help.
The first time Bruce shows up at his apartment, drenched in rain, begging Clark to help him find Green Lantern because an unknown element in his plan was tampered with, and Hal in injured in an unknown location, Clark's heart goes wild and blood rushes through his body. The first time Bruce shows up to his quarters in the Watchtower with food to discuss a case, he smiles to himself. His plan was working.
-- Insert picture of Clark giggling while staring at his bulletin board of all his plans and pictures and sticky notes with random things he's learned about Bruce --
extra stuff on this, but I don't know if I should write longer entries...