Register new Instagram application and note your Client ID, Redirect URI and copy&paste following code with replaced CLIENTID and REDIRECT URI to your browser.
Instagram will return Access Token to your browser Address Bar with following code:
https://www.instagram.com/oauth/authorize/?client_id=CLIENTID&redirect_uri=REDIRECTURI&response_type=tokenInstagram will return Access Token to your browser Address Bar with following code:
REDIRECTURI#access_token=YOURACCESSTOKENCopy&Paste Access Token to Customizer Social Section.