These instructions assume a linux|unix type system. Similar steps apply for MS Windows users.

To use the syntax file copy it into the directory ~/.vim/syntax (create the direcory path if not already present).

Create a file named red.vim in ~/.vim/ftdetect (again creating the directory if not already present).
This file defines .red as the filetype of REDCODE source files. It should contain:

                augroup filetypedetect
                au BufRead,BufNewFile *.red setf redcode
                augroup END
            

The following options can be used in your .vimrc initialization file to control the degree of syntax checking:
They are all commented out by default.

                "Remove quote characters on subsequent lines to enable options

                "let redcode_88_only = 1
                "let redcode_94_only = 1
                "let redcode_highlight_numbers=1
            

The default behaviour is to accept '88 '94 and Pspace syntax and to not highlight numbers.