Start with the instructions here, but they don't cover the use of EFI/GPT volumes.  So when you're frustrated with that article, check out these steps.

Install the system on to one disk.

Normally, you would just add another volume with a command like this (where c7t1d0p1 is the disk you are planning to add to the rpool):

zpool attach rpool c7t0d0s0 c7t1d0p1

However, if you are using EFI, you may get an error like this:

root@server:/home/michael# zpool attach rpool c7t0d0s0 c7t1d0p1 Error: c7t1d0p1 is part of an EFI(GPT) labeled device. A required boot partition (BIOS Boot) was not found.

In this case, create a new 256MB partition on your second disk, and in parted, run the following command to make it a bios_boot volume (where 2 is the partition number):

(parted) set 2 bios_grub on

Now, re-run the following command, and you should be set:

zpool attach rpool c7t0d0s0 c7t1d0p1

Make sure to wait for the reslivering to complete.