You first need an application which your customer must authorize to make requests in his behalf.
That application will first need to be whitelisted for Ads API access, by a Twitter Representative.
Once you have this, you need to implement the login flow (found in the docs) by which you get access tokens for the user (the customer), and then you can make requests to the Ads API to get whatever data you need.
To make valid requests you need to send the “Authorization” request header - you can do this manually (the steps are documented), or use an SDK that does this for you.
Good luck! 
