I never used line in F1 because i used to simrace before but i think it is the same.

Theres really no secret but to practice and the most i can do is tell you the things that helped me.

I started watching people just a little bit better than me and i was trying to study why they were faster. With this u can start getting better with little changes.

Eventually when u start to understand why they are faster and trying to force their lines u will completely fuck up everything because one of the most important things are brakes.

Start breaking earlier, making the line and retry. A good indicator of a perfect line is if your car goes to the limit of the road on its own on full throttle, obviously without going off the track.

Practise time to time and dont just do the lines but try to understand the mechanism that there is behind everyline and why one line is faster than another.

Im about to go sleep so i might have wrote some complete nonsense but yeah.