These are the same 3 methods you see in a menu.lst, except in menu.lst you have "title" lines and you don't need the boot command (since it's implied).
Method #1: Chainload To chainload Linux which is on (hdx,y):
grub> root (hdx,y) grub> chainloader +1 grub> boot
Note: Windows is always booted by chainloading Note: To chainload, GRUB must first be installed in the OS's root partition. See Installing GRUB. So, run the commands:
grub> root (hdx,y), grub> setup (hdx,y)
Method #2: Use the specific kernel-initrd commands (as in a full menu.lst), ending with the boot command.
Method #3: Configfile This almost always works! Example: To boot into Kubuntu on sdb3 (= (hd1,2)), type
grub> configfile (hd1,2)/boot/grub/menu.lst # you'll get the boot menu on sdb3