Do all of the following:
Stop playing ARAM.
Stop playing Twisted Treeline.
Stop playing Normals.
Stop playing Riven.
--
Pick one of the following:
Main Hecarim top.
Main Irelia top.
If you picked Hecarim -> http://www.twitch.tv/keanelol/profile/past_broadcasts -> watch 2-3 VODs of Keane playing Hecarim per day.
If you picked Irelia -> http://www.twitch.tv/ireliacarriesu/profile/past_broadcasts -> watch 2-3 VODs of IreliaCarriesU playing Irelia per day.
--
Do all of the following:
Spam ranked on your selected champion.
Practice last hitting with your selected champion in a 10 minute long custom, 2-3 times per day.
Set up custom game 1v1s (Summoner's Rift) and play against anyone who comes in.
Watch every video on Crumbzz's youtube channel: https://www.youtube.com/channel/UCm05X6pqXnwUgZBa-kdr-cg/videos
--
In the current meta, it is highly unlikely that you will be unable to climb if you follow the above steps for a few weeks. Whether you care enough about climbing to actually commit to this, I don't know.