Poured concrete footer for French door

Hello, I am in the process of refinishing a basement. I will be replacing a garage door with a French door and will need to pour a concrete footer at the edge of the slab for the door’s threshold to mount to. Currently, the slab ends inside the door opening which is fine since the garage door mounts inside the wall.
My questions are as follows:
1. How deep should the footer be? I’m planning to go as wide as the wall and tall enough to accommodate future floor leveling where the basement floor slopes down at the current garage door opening.
2. Since I’ll be leveling the floor as mentioned in the first question: should I pour that I’m at the same time as the footer or should it be a separate pour.
3. What questions should I be asking that I’m not?
Thanks in advance!
-Matt