How do I load a kernel?

You can load a kernel image by the command @command{kernel} and then run the command @command{boot}. If the kernel requires some parameters, just append the parameters to @command{kernel}, after the file name of the kernel.

How do I load a kernel first?

Change /dev/sda2 with your root partition , change gpt2 with msdos if you don’t have a GUID partition table. You have to define a kernel file and usually an initrd file and the kernel command line, too, before you can run the boot command (see the Grub documentation).