USB boot drive question

Hi,

I'm changing my hard drive in my late 2011 Mbp to a SSD. I'm also taking the dvd drive out and putting a new hdd in place.

Currently I'm on OS X 10.8 but I want to install OS X 9.

I've read some guides on how to make a USB boot disk, that seems ok, but my question is, when i install the new SSD the disk will be totally blank, so how do I go about installing osx9 onto the SSD?
Is it a case of just putting in the USB drive, powering up and the map sees the installer on the USB? Or?......

Do I need to hold any keys down when I boot it up? And do I need to format the SSD drive first?

Thanks for any help and advice