Here is an easy way to label campaigns, Ad groups, Keywords and Ads using Adwords Editor.
To start Power up Adwords Editor and import all changes
Select the Campaign, Ad group, Keyword or Ad you want to edit using the left console. Once you have done this select Labels in the lower console and select Edit
After this you can then create a new label
Click the new “Label Sign” and name the label
When done just assign the label to the relevant entity.