Speaking for myself, I find this process to be one of the most enjoyable parts in writing. When creating characters to populate my imaginary worlds I follow three steps to make them both well rounded and enjoyable for the reader. All three steps can be used for the protagonist/antagonist all the way to minor characters that appear in one scene/chapter, the only difference between them being the amount of detail invested in each. While putting this information in a notebook or on notecards can be useful to have the characters at my fingertips, you can also type it up in a word doc as backup just in case. I also use a flash drive and external hard drive to make sure nothing is accidently deleted.
Component 1: Background and History
Start out with the name of the character, general personal/family history, where they live, and information like their station in society, any relationships they might have, as well as abilities and skills. Magic abilities would also fall in this section, though try to avoid getting too elaborate here as it will distract from the character you are creating. Instead save that for world-building.
Component 2: Motivations and Goals
In this part, think about what drives my character in the story. Knowing what the character wants makes it easier to write a story that follows a convincing, logical progression. By doing this doesn’t mean that you won’t be able to do plot twists or change the direction of the story as you write, just that each character’s actions should make sense, given what the reader knows about them.
Component 3: Appearance and Personality
Lastly is how to describe your characters physical characteristics and personality. For the first part this means not only basic things like eye color, hair color, height and weight, but also stuff like scars and typical garb/armor/weapons. In terms of personality, you can start out with single word descriptions to get a feel for the character. For example, you might start with ‘honorable’ or ‘deceitful’ and build upon those to make a complete personality profile.
For my main characters in my writing each part tends to be relatively in depth, whereas for minor characters I tend to be more brief. If you follow something like this approach I recommend that you should still have something for each character you write about, if only to add some depth to characters that would otherwise might appear like placeholders. It doesn’t even need to be overtly stated for it to shape how you write a character after all. For example, who would you rather read about interacting with your main hero as he comes to a new town at night, the night gate guard of indeterminate description or the grizzled guardsmen veteran who stands out beyond the protective wall in the cold night out of a sense of honor and protectiveness for his town and his family? Personally I’d pick the second every time.