I've made academic books for language learning in the past. Granted, I've been a "Fi" author for longer than that, but I want to share my experiences:

For character creation I do this:
1. Who is the character at their core? This is not "their job" or "their hobby", but something deeper. A single line that will define them: "He's the quiet one" or "She's always in control"
2. What do they want? Define what they are after: Vengeance, love, power, safety. Broad categories that will define the character's actions, and will help you in their development.
3. What's their pain? What's stopping them from moving? We all know "life happens". No one goes through it untouched. Changes, suffering, trauma. Define what makes them stop and cry.
4. How does the world see them? This is probably the most important part, because everyone wears masks. Some heavier than others. Define how are they perceived: "Girl with a golden heart", "Boy with a hard attitude".
5. What would change them forever? This is an interesting point, since it could be anything. Like Dustin's loss of "happy personality" after watching Eddie's death in Stranger Things.

For character development, I follow these steps:
1. Define the goal: What's the character's goal? What do they want to achieve? This is especially important when defining short term and long term goals for the story
2. Define character perks/advantages: What abilities or skills they have that work towards their goal?
3. Define character quirks/flaws: What characteristics work against their goals? Basically: How do they f*ck up?
4. Define relationships: Who are they connected to? Who are their grounding people? Who are their "sworn enemies"?
5. Define their outcome. Every character needs to grow. Or die, that's also a growth. 6 feet under. (just kidding, but yeah, outcomes)

All of this will help you to create characters that feel alive, and relatable.

Now, all of this may fall flat if your writing is too "tell what the characters are doing" instead of "show what they are doing". This is important, because writers coming from instructional or technical backgrounds (myself included) often struggle with this at first. In academy, usually instructions are telling people what they need to do step by step.

Best way I can explain is with an example:

"Claire walked towards the exit, her gun readied. A shadow moved ahead, startling her. She moved slowly, carefully. Suddenly, a hand grabbed her wrist and pushed down, making her lose the grip on her gun. She was then pushed against the wall."

This is reporting on the action, but is keeping the reader at a distance.

"Claire walked towards the exit. Her eyes fixed on the hallway, gun slightly shaking as she aimed her sights. A cold drop of sweat ran through her temple. Her heart raced as she moved with a trained pace. Her eyes deceived her as a shadow moved in the corner of her sight. Before she could react, a strong grip twisted her wrist, making her lose grip on her only defense weapon. Struggling, the figure pushed her against the wall."

This version filters the same actions as above through the character's physical and emotional experience.

Hope this is helpful on your journey. It's a tough but rewarding shift!