There is no real confidence without experience. What we know without hesitation is the outcome of experience. Everything else is guessing.
To get experience you have to do the thing, the whole thing. You have to go from A to Z, from here to there, from beginning to end. And when you do, you know more, and you are secure in the knowledge of what you know.
Read about fire. Think about fire. Look at pictures of fire. Draw images of fire. Do you know fire?
Stick your hand in fire. You know fire.
Stick a raw potato in fire. Eat a cooked potato. You know fire.
If someone asks, Should I stick my hand in fire?, will you be insecure telling them no? If someone asks, Will fire cook food?, will you wonder if it does? Of course not.
That’s the confidence that comes from experience.