Apply gitignore to existing Git repository

Problem

We have existing git repository which contains e.g. .net projects written in C# and we want to apply gitignore rules.

Solution
At the beginning we need to crate our own ignore rules or download predefined from https://github.com/github/gitignore.
In this case I prefer download predefined file: VisualStudio.gitignore and put in my existing git repository.

Don't forget commit all pending changes. After that run below commands

git rm -r --cached .

Below command removes everything from the index

git add .
git commit -m ".gitignore is now working"