Excerpt for the first post

Hi,

Thank you very much for this beautiful theme.

I just saw this thread : https://wordpress.org/support/topic/bushwick-hides-nearly-everything-about-posts-on-the-blog-page/ > it is quite I need.

I would like to display an excerpt of 50 words of the first post instead of the entire content.
And same for next posts after in the list.

How can I do that ?

Thank you very much.