Step 1:
Start off with looks, What does He/she look like? What do they wear? What gender are they?

Step 2:
What is their personality? Are they Good? Evil? Sad? Happy?

Step 3:
Perfection is a flaw, perfection is a one way trip to a Mary Sue. Create a few flaws here and there, like give your character a disorder or disability. My first character ever used to be a Mary Sue, but I changed it up a little bit, and I still am changing it.

Step 4:
All characters want something, whether it be a glass of water, or to bring peace to the world.

Step 5:
How will they get what they want? Why do they want it? Don't put "because they are just that way," as an answer. These simple questions open up the paths in life. There is also the questions of "when do I want it? Where?" This opens up more opportunities for this character.

Step 6:
What compromises will/did they encounter? Like "there is a fork in the forest path, one leads to a dark place while the other will lead to a happier place." Which one will your character take?

Step 7:
Origin story? What are they? Where were they born? What hardships did they overcome in life before the main story takes place? The reader doesn't even need to know it all, but it's useful to have when you need something to go off of. My character, for example was a victim of war and lost her mother. Just remember, it's better to put bits and pieces here and there, than to write it all as one big flashback. Unless the story you are writing is that character's backstory.
You can base some characters off of yourself, but not all of them. It's just better that way.

Step 8:
Quirks. What kind of quirks do they have? Do they have a volcal tick? Or do they use hand gestures while talking. Quirks make a person who they are, ya know. My character, which is a dragon for your information, has double jointed wings, and folds her wings in half. Just because she doesn't like others seeing her under-wings.

Step 9:
Ask simple questions about your character, like "favorite food? Color? Age? Height? Wish in life?"
My character's favorite color is red, doesn't have a favorite food, 7153 years old, and I'm not even gonna get started on height... Her wish in in life is for there to be no more rivalry between her and her enemies.

Step 10:
Your finished, but your not... People change all the time. Personalities change. Never consider your character finished. I made like 1827383 backstories for my character. But I think I've settled on one. It still depends on what fandom I'm entering her into. Sometimes she is not even in a fandom. Sometimes she's just there to advance the plotline. It honestly depends on your opinion how your character will change and why.

Let's review!:

Looks?
Personality?
Flaws?
What do they want?
How will they get it, why do they want it? When and where do I want it?
What will they do to get it?
Why do they do what they do? What is their past?
What cool/funny quirks do they have?
Ask the simple questions.
People change, ya know.

My character:

Name: StarShifter

Age: 7153

Height: depends on what form she's in

Likes: family, friends, what little peace is in her mind, etc...

Dislikes: enemies, people who piss her off, being annoyed repeatedly, people who insult her family, etc...

Allies: her kingdom (she is a queen, and a good one at that.) anyone who she considers a friend

Quirks: selective mutism, blind, black scar on left side of face, etc...

Flaws: can be intimidating, and shy at times, inability to feel MOST emotions, etc...

There is a lot more to StarShifter, that what there is here.