Hi'

I am a beginner. I put the index finger down first to establish the initial fret position and then the other fingrers. As you get more proficient you will find that the interval between putting the index finger down first then the other fingers will get progressively shorter until the time period disappears. This takes time of course. Just practice and practice. You will get there.

Don't forget your posture. Correct posture makes barre chords a bit easier.

Pay atention, stay focused. Eventually it becomes automatic. Like driving a car.

I wish you success.

Joe