Still waiting for ITEM SETS

They can't work on it until the new client has completely replaced the legacy one. They are dedicated to doing so, however. In the meantime, you can use this online item set creator. (or set them up in the legacy client before the 24th)

In the meantime, use this approved 3rd party version to manually insert item sets into the client. If you want confirmation that it is safe or allowed, there was a top upvoted post debuting it that you can search for on boards (with Riot comments).