How to make a good comic book movie
1. Pick a character.
Pick a character or characters that have sustained an ongoing series for at least a dozen years. Longevity is a decent indicator of a character's star power. You'll want a built-in fanbase, or at least a bunch of people with fond memories of it even if they aren't all going to go on opening night. You'll also want a ton of backstory and side plots to reference and sift and pull together into one coherent narrative.
Examples: Basically every successful comic book movie did this and a lot of failed ones; it's easier to list exceptions. The comic books The Men in Black was based on lasted less than two years. The Mask was published irregularly for less than four years before the movie. In general, moviemakers seem to get away with movies based on comics with shorter runs if they weren't superhero comics, like almost anything by Alan Moore, who famously hates adaptations of his work.
2. The origin story.
When you think about it, this is more of a genre convention in superhero media than in other media. No one knows or really cares where James Bond comes from or why he does what he does, Bram Stoker didn't show us Abraham Van Helsing's first encounter with a vampire, Aragorn is one-dimensional, and Sherlock Holmes had been a detective for years when Watson and the reader first encountered him. But every comic book character on the movie screen gets an explanation on-screen of when and where and why and how they stopped being a normal person. Maybe it's because comic book characters tend to be so outlandish that the explanation is really needed.
Update the origin story as needed. Nuclear power has been around for 60 years by now with a conspicuous lack of superpowers. It does not make a few hundred pounds of biomass appear out of thin air, like apparently happens to Bruce Banner. As for genetic engineering or nanotechnology, on the other hand, well, who knows? Also, simplify it. In comic books, aliens, wizards and mutants interact all the time, but that's apparently taboo in movies.
Examples: except for sequels, I can't think of a movie that doesn't feature the origin story, and most of them try to tie it into what the character is doing now. The 2008 Hulk movie might be the closest there is, in which the origin is only shown as a dialogue-free flashback during the credits. As for simplifying, in the comic books, the Phoenix is a cosmic force that possessed and/or cloned a mutant during a near-death experience and lived as her for a while (it's complicated), and the Juggernaut is a guy wielding the power of a demigod of destruction. In the X-Men movie, on the other hand, they are just particularly powerful mutants.
3. Pick a villain.
This is more complicated than it sounds. Many comic book characters are best associated with villains that don't fit their own "theme"; see above about aliens, wizards and mutants interacting. Some comic book characters have really cool, interesting rogues galleries of their own while others are best associated with guys called Stilt-Man and the Owl. Also, it is apparently mandatory to work the villain into the superhero's origin. The fact that he can shoot lightning from his hands is not enough reason to feel threatened by a megalomaniacal sociopath, and the fact that he's planning to kill the Eastern seaboard for personal profit is not enough reason to hate him; he must also have killed the hero's parents, given the hero's best friend a lifetime's worth of psychoses, or at the very least caused the hero to lose his previous job. This is probably because the movie industry is higher-stakes. If you're spending hundreds of millions of dollars on a movie that's not guaranteed to succeed, you want every possible ounce of conflict and pathos and angst you can get.
Examples: Superman gets away with a villain in the movies with no ties to his origin (and even in his case, the villains of Superman 2 are Kryptonians), but everyone else needs them. In the comics, Spider-Man encountered Electro, the Vulture and Dr. Octopus before the Green Goblin, but the Goblin is Spider-Man's most personal foe, so he's the one in the first movie. In the comics, Kingpin first appeared as a Spider-Man villain, but in the Daredevil movie not only is he Daredevil's first big enemy, he's connected to Daredevil's origin like the Joker to Batman. (The Joker to Tim Burton's Batman, that is, because that's another example of this kind of thing.)
4. Continuity Nods: include them.
This is part of why you want a long-running comic, so you have more to draw on. In addition to minor characters or scenes or plot points from the comics, you can also include nods to the creators of the comic, by naming characters after the writer or artist. It'll make the fans go "squee" and gush to their non-fan friends they dragged along to the movie. Hopefully you'll be able to use it as a Chekhov's Gun in a later movie in the series, but if not, no big deal. Incidentally, it also lets you include more character actors in meaningful roles and make bit parts memorable. Absolutely no one would ever remember the name of the assistant police constables in the 2009 Sherlock Holmes movie, but in the X-Men movies practically every extra in a school scene was named after someone in the comic, and most of them have decades of history.
Examples: Like I said, they're practically omnipresent. A couple are noteworthy, though. In Tim Burton's Batman, do you remember the cameo appearance of Harvey Dent, the villain of Batman Forever? No? Probably not, because in Batman he was played by Billy Dee Williams, not Tommy Lee Jones. (That's actually kind of appropriate, if you think about it, but anyways.) Likewise, Gambit in X-Men Origins: Wolverine and Curt Connors in Spider-Man 2 were fun to see, but actually made it harder to give those characters bigger roles in later installments.