Grab the grub source code from www.gnu.org/software/grub.
Compile it using the standard method (./configure, make, make install). Then run grub-install and grub to set things up. There is a guide on how to do everything in the documentation folder that uncompressed with the rest of the grub files. It tells you all the commands, how to use them, and how to set up a boot menu. It also explains how to boot various OS's including Linux and Windows using grub.