Start Terminal and create the ~/.ssh directory if it doesn’t exist:
mkdir -p ~/.ssh
Now go into the directory:
cd ~/.ssh
Now open the file config with the default editor:
open -t config
If it says that file doesn’t exist, create it with:
touch config
and try opening again:
open -t config
When you edit the file, make sure it is “Plain Text” using the menu at the top of the screen and clicking Format->Make Plain Text.
Set the permissions as instructed in your tutorial.