You need to set up an account here: https://www.google.com/adwords/manager-accounts/
Here are the steps, for the most part. It's detailed in the above video.
1) Use a different email account
2) Select the option to Manage Other People's Accounts
4) Notice that I skipped point 3
5) Requestion permission from this new account to connect to your old account.
6) Log into your old account, Go to Gear > Account Settings > Account Access and grant permission.
7) Log back into the new account and select the old account from the drop down menu.
7) Start using the Keyword Planner
8) Notice that I Just Repeated 7
9) Now you learn that I used B instead of 6 above
10) Now you went back to check. Gotcha!
