If it wasn’t already clear, the Planche is sort of the holy grail of bodyweight training skills. It’s extremely difficult to achieve, on par with an iron cross. There are lots of conditioning and strength building exercises one can do of course, but mainly it just takes time!
I am making some good progress. This pic below shows rock solid straight arms, with knees just behind my elbows. Once I can hold this position for 60 seconds, I’ll be ready to extend one leg out further, or try a more opened position. This position pictured is called a ball planche:
If you’re working this skill I recommend you get a set of parallettes. Here’s a good article on various skills to work on with parallettes.
Dragon Door also has a very good article on planche progressions which I recommend.