One particular bit of advice is about character outlines. These are basically a form-based description of a character, something like the following:
Full Name: Marie Kockaert
Nick Name(s): Moonbeam
Age: 400 or so years old, turned into a vampire at 17
Marital Status/Sexual Preference: unmarried, probably heterosexual but too precoccupied to think about it
Language(s): English, French, Flemish
Height: kinda short but not unusually so
Weight/Body Structure/Physical Faults: very skinny
Physical/Mental Health: believes drinking blood is bad so is constantly starving herself, accident prone due to lack of concern for own well-being, extreme black-and-white thinker, obsessively religious, suffers from unrecognized PTSD due to the Spanish Inquisition, feels like she should punish herself, longs to die but thinks suicide is a sin
Clothing: she usually just puts on whatever, though she avoids revealing clothes because she considers them sinfull
Parents/Guardians: she was a farmgirl with Catholic parents who had mixed feelings about the Spanish invading (at first rooting for them in their efforts to rid Belgium of heretics, then more and more alarmed at how far they were taking it). Her vampire parent was Bjorn, an Icelander who believes in honor, kinship and raiding/pillaging, and who agreed to turn her in exchange for sparing her family after his son was killed
Siblings: an older brother, who killed Bjorn's son and was killed himself in revenge
Usually they're much longer, but you get the point there. There are two potential purposes I can see for such an outline - to keep from forgetting little details (eg having your character's eyes brown in chapter 1 and blue in chapter 3), and to flesh out your character. The first is a pretty good reason for an outline, at least containing any such niggling details. The second, though, is a very bad idea.
See, what character outlines do is chop the character into a bunch of little pieces. It's very tricky, when looking at a character outline, to pull those together into a coherent whole. If you already have a good sense of the character, an outline won't hurt, but it won't really help either. But if you don't have a clear idea of them, you'll end up with a bunch of disconnected pieces and no easy way to turn these into a real character. At worst, you may have contradictory traits, such as describing one of a character's strengths as 'easy-going' and one of their weaknesses as 'hot-tempered'.
But the reason these are so widely appealing, I know, is that they give struggling writers something concrete to do to flesh out a character. So I'm going to offer something concrete for a writer to do if they need to flesh out a character, that should give you someone much more well-rounded than a character outline will provide.
Gordon Allport, one of the early personality theorists, suggested that personality traits can be divided into three main categories - cardinal dispositions, central dispositions and secondary dispositions. Secondary dispositions are minor, unimportant traits, more akin to habits, such as a student who always chews on his/her pencil when pondering a multiple choice answer. People have an uncountable number of secondary dispositions. Central dispositions are relevant to multiple situations and show up in multiple different behaviors, but are not the most crucial traits to a person's personality; typically you can identify 5 to 10 of these. And lastly, cardinal dispositions are the defining traits that tell you who this person truly is, and each person has only one or two of these.
In sketching out your character, start by identifying their cardinal traits. For example, Moonbeam's cardinal traits are her self-hatred and her fanatical Catholicism. Just knowing that about her tells you a lot. (Of course you'd need to know that she's a vampire, too, but that's not a personality trait, it's a background detail.)
Incidentally, if you have no clue what your character's cardinal traits should be, you can look at your intended plot for ideas. Their personality will shape their actions, and their actions will shape their plot, so identify specific things your character has to do in order for the plot to unfold as intended, and figure out what personality traits might lead them to do that. For example, one character in a different story, Terald, has to help a friend of his murder her villainous sister, then once he discovers the sister alive and severely brain-damaged, he has to spare her and take care of her. This led me to the realization that he's an extremely forgiving and highly moral person who has been sorely tested by this villainous sister, and reluctantly decides that he needs to take drastic measures to ensure that she will cause no further harm. As a result, I realized that Terald's cardinal dispositions are his strong moral code and his logical reasoning, which are often pitted in conflict with each other. (Note that if you have to chose between a well-fleshed-out character and a desired plot, I'd pick the character first. Only go from plot to character if you don't have a fleshed-out character yet.)
Once you've identified the cardinal dispositions, flesh them out. You can do this is two directions. Firstly, go back in time. Where did this trait originate? Is it part of their inborn temperament, or was it shaped by their environment? In Moonbeam's case, she was Catholic but not a fanatic as a girl, and then after a couple decades of being a vampire, she was tortured by the Spanish Inquisition. She decided they were in the right, and this belief led to both of her cardinal traits. (Note that formative events can change cardinal traits, but this is tricky to do convincingly in-story.)
Secondly, look at this cardinal trait in different situations. What specific thoughts, feelings and behaviors are involved? In the case of Moonbeam's self-hatred, this means painting out what, exactly, Moonbeam hates about herself. She hates that she's a vampire. She hates that she's hurt people. She hates that she's immortal, which to her means being cut off from God and from the natural order. She hates her unacknowledged anger and fear at the torturers, and love (parent-child love, by the way) for Bjorn. She hates that she's been miserable for so long. In terms of her fanatic Catholicism, this would mean deciding what, specifically, she believes, but you get my point already.
At this point, chances are you already have some central dispositions. Moonbeam, for example, has shown in this bit that she is quite determined and doesn't flinch away from unpleasantness, because rather than trying to forget her sins she's trying to atone for them. She's also a caring person, as shown by her concerns about hurting other people. She is highly anxious, as a result of her traumatic experiences. All of those come directly from fleshing out her cardinal traits.
Then, you can add other things which fit well with both your existing characterization and your intended plot. For example, since Moonbeam's role in the plot is as a supernatural detective, she has to be an intelligent person - a stupid person wouldn't be much good as a detective.
Around this point, you probably have thought of secondary dispositions too. But don't put too much effort into fleshing these out, unless they're plot important or add realism to the character. For example, Moonbeam's shoulders always start to hurt whenever she prays, which is a sensory flashback to a particular torture technique that tends to dislocate the victim's shoulders. This detail is included mainly as one of several hints leading to the eventual reveal that she was severely traumatized. Another kind of plot important secondary disposition is one that causes someone to recognize them when they don't want to be recognized, such as when they're disguised - the brain-injured villain in Terald's story is eventually recognized by her sister due to her favorite food and her love of climbing (she used to climb things all the time as a small child and eventually outgrew this behavior, only for it to re-emerge when she's at the same cognitive level once again). But you won't be able to flesh out every single secondary disposition, so don't even try.
I hope people find this helpful.