Step 1: obtain template

Click the link and go to the file menu of Google Sheets and click 'USE TEMPLATE' in the upper right hand corner

Go to Template

Step 2: sign up for Blizzard Dev portal

Go to https://develop.battle.net/ and click on Get Started Now

Step 3: Create a client

Just click the button!
IT CAN TAKE UP TO 10 MINUTES FOR YOUR DEV ACCOUNT CREDENTIALS TO START WORKING! (and sometimes longer, thanks blizz!)

Step 4: Fill out the form

Name your client, doesn't really matter what you choose.
Leave the next box blank, you do not need any redirects.
Check the box next to I do not have a service URL for this client
Type in your intended use for your key, which is a google spreadsheet.

Step 5: Reveal your secret

Click on SHOW SECRET
You'll now see your secret

Step 6: Enter your API info

Put your Api Client ID into the settings sheet cell E18
Put your Api Client Secret into the settings sheet cell E19
You can change any of the other settings you want in Section (B) as well

Step 7: Save your values

Once you have entered your Client ID/Secret, type into sell E1 to save the settings to the script.

Step 8: super important.. DELETE!

You will then need to delete the your ID/secret from section (B)'s "New Values" column, to keep things secret!