The promised Practice Tool for players looking to hone their League of Legends [official site] skills (and thus turn everyone into the next Faker because, really, access to a Practice Tool was the only thing standing between me and him) is now available on updated game clients and has an excellent announcement trailer.
You can find the tool under the Training tab once the client is updated. It's a solo mode where you can either roam the Rift alone or add a bot player on the opposing team so you can't co-opt it for 1v1 URF nonsense with your flesh and blood buddies (not yet anyway).
The tool itself lets you do the sorts of tweaking you'd expect. So you can fiddle with your character's level, dish out gold in order to buy items for build test runs, reset jungle camps, reset ability cooldowns, spawn allied and enemy dummy fighters, teleport to locations - essentially everything you'd need to set up and then practice particular scenarios. I've made a super-quick video to show the hover-over tips for each of the options in case you want the full list.