How does one start a 'practice mode' session?

I have seen many posts on various riot and non-riot sites talking about the 'practice mode', many of which feature detailed descriptions and video of the game mode. A few posts say that the mode was added in 7.3, and a couple say that to launch a practice-mode game you need to launch the 'new legacy client'. When I launch the new (and gorgeous) client, as well as the legacy client, I cannot find the practice mode at all.

My question should be obvious- How does one start a practice mode game? Do I need some kind of beta client? Google is woefully inadequate in providing an answer at the moment.

Thanks for helping me get my last-hit game up <3