To configure bash aliases, it’s the same as if you were on a Unix platform: put them in a .bashrc in your home:
cd
echo alias ll=\'ls -l\' >> .bashrc
To have this change taken into account you should then either source this file (ie: run source .bashrc) or restart your terminal
(In some cases* you can find equivalent for .bashrc file in C:\Users\<username>\AppData\Local\GitHub\PortableGit_\etc\profile.d\aliases.sh. And you should add aliases in aliases.sh.)
(*this case is when you install Git for Windows GUI release from https://git-scm.com/download/win that contains GitBash)