When you installed a second kernel, you copied the bzImage to /boot/vmlinuz.kernel_version or similar. You have also created an initrd image using the mkinitrd command.
Now all you have to do is edit the /boot/grub/grub.conf file accordingly. Take the lines that are in the file already, copy them and change accordingly: (example below)
title GNU/Linux Fedora Core 1 (2.4.22-1.2174.nptl)
kernel /vmlinuz-2.4.22-1.2174.nptl ro root=LABEL=/ hdc=ide-scsi apm=off acpi=on rhgb
This is similar to what you might have. Now to change it you would only have to copy it and make it look something like this:
title GNU/Linux Fedora Core 1 (2.6 custom made)
kernel /vmlinuz-2.6 ro root=LABEL=/ hdc=ide-scsi apm=off acpi=on rhgb
Where vmlniuz-2.6 would be the actuall name you named your new kernel while copying the bzImage file to the boot directory. And initrd-2.6.img would be the name you specified as the initrd image file.