To translate your *nix style command script to windows/command batch style it would go like this:
SET PROJ_HOME=%USERPROFILE%/proj/111
SET PROJECT_BASEDIR=%PROJ_HOME%/exercises/ex1
mkdir "%PROJ_HOME%"
mkdir on windows doens't have a -p parameter : from the MKDIR /? help:
MKDIR creates any intermediate directories in the path, if needed.
which basically is what mkdir -p (or --parents for purists) on *nix does, as taken from the man guide