how to add gitignore to existing repo code example

Example 1: git add gitignore

$ touch .gitignore

Example 2: Force add despite the .gitignore file

git add --force my/ignore/file.foo

Example 3: apply gitignore to current commit

#First commit all changes
git rm -r --cached .
git add .
git commit -m ".gitignore updated"

Example 4: what to add to gitignore

Look at this awesome site!
https://www.toptal.com/developers/gitignore
Generates a gitignore for you, based on the technologies you're using