Feminist theory covers a lot of ground and a lot of approaches to literature. The simplest way to understand feminist criticism is to say that a feminist reading of a text is one that focuses on women.
Sometimes that means focusing on books that were written by women--often as a way of correcting a historical inequality. For most of the history of literature most of the books that have been seriously studied have been written by men--but men were never the only people writing. Feminist criticism is a way to try to fix what we read and who wrote it. This kind of feminist reading would point out that there have proportionally been very few women who write comics for DC, but would draw our attention to women like Dorothy Woolfolk, Mindy Newell, and Gail Simone.
Sometimes that means focusing on female characters in stories--again often as a way of correcting historical inequality. Focusing on female characters in a feminist context can mean a lot of different things, but in brief it always has to mean focusing on the female characters themselves, instead of just focusing on how they relate to male characters. So talking about Lois Lane is a start but if we're talking about Superman's girlfriend Lois Lane then we're still defining her as a character in terms of her relationship to a male character. A feminist reading might instead want to talk about Lois Lane herself. Or even a focus on Superman can be feminist in this way if it is a focus on Superman as Lois Lane's boyfriend.
Sometimes that means focusing on gender in general. Certain strains of feminist theory have stressed that "woman" as a category is invented by culture, and that it is defined in contradiction to "man" as a category. So a reading of Superman that focuses on how Superman is a representative of masculinity, especially when that reading emphasizes the way masculinity is socially constructed and the way that it simultaneously constructs femininity, might also be a feminist reading.
Those are only three of many ways to do feminist criticism, but I hope you are getting a picture of how much scope there is in feminist criticism. I'll be back soon with a second post on feminist criticism, where we will actually do a (brief) feminist reading of Action Comics 1.