Install to external (I think) SSD

[Grizzly]

Hi all

I'm trying to show the advantage of Ubuntu (Xenial Xerus) to a friend who only 
knows iBook (OS X Yosemite)

We made a live USB (64 bit) and that runs fine, bit he fears that as the 
initial (grub ?) screen has such small text he could install over his OS and 
also the Install icon wont stay away once up & running, so he wants to install 
to an (external) SSD 

Is this possible? and can it be done from the "normal" live USB install or does 
it need special treatment

I run a lot of ububntu version sbut so far have not playerd with partitioning 
leaving that to the USB installer

Any help please, I will be trying top do this later today