Start here.  This is a good example.
https://www.codeproject.com/Tips/277847/How-to-use-Cursor-in-Sql

Now read and comprehend this:
https://docs.microsoft.com/en-us/sql/relational-databases/cursors

Here are some references:
https://docs.microsoft.com/en-us/sql/t-sql/language-elements/cursors-transact-sql

And then:
http://bradsruminations.blogspot.com/2010/05/truth-about-cursors-part-1.html