(MacOS Mojave Example)
Go to your home directory:
$ cd $HOME
Then open the hidden .bash_profile file in the vi editor:
$ vim .bash_profile
Jump to the last character of the file by pressing capital G and then $
Press I to be able to insert text in the vi editor
insert your new alias like:
$ alias ll='ls -lG'
press control + c to quit insert mode of vi
type following to write and quite your new changes:
$ :wq
restart your terminal
reopen terminal and you should be able to use the alias ll