Hi.
The Netlify docs have a tutorial on setting up the CMS. You have to make an admin HTML page with some configuration and then when deployed you can login to the CMS on the site.
And if you find the Netlify account on GitHub, they have a one click deploy template for a site using Netlify CMS.
I did have issues using it before as there is a lot of code on site and the admin view. So building from scratch might be more useful.
A codeless approach is to use Forestry. It works for Jekyll, Hugo and more. You can generate a new site in Forestry
Or import your Jekyll site.
It needs some set up sometimes but you can manage some complex stuff in Forestry like required fields in the frontmatter of a page or a list of key value pairs. And when you save, it writes to your repo but it much simpler than Netlify.
I wrote a post on it