1. Ensure there's a syntax directory in the personal vim directory.

     mkdir -p ~/.vim/syntax
  2. Download the markdown syntax file and put it in this directory (e.g. markdown.vim).

  3. Affect the markdown filetype to *.mdwn files and avoid long lines (in ~/.vimrc).

     " Help autodetection
     augroup filetypedetect
         au! BufRead,BufNewFile *.mdwn setfiletype markdown
     augroup END
     " Limit the line length for this mode
     autocmd FileType markdown set tw=80
  4. Enjoy.