First of all login to Instagram then go to https://www.instagram.com/developer click on "Register Your Application". See the image below

Then register as a developer and fill the form and press Sign up. See the image below

Then Register a new Client, Fill up the form with real data with your URL (Where you use it). Then Click on Register. See the image below

Now you are on Details Tab. Go to Security tab uncheck the "Disable implicit OAuth" option. and click on update client from below. 

Then On top you get Client ID, Client Secret, Client Status the status is always in "Sandbox Mode"

Now go this link with your data https://www.instagram.com/oauth/authorize/?client_id=Your_client_id&redirect_uri=Your_site_URL&response_type=token&scope=public_content

And go to this link. You will Redirect to your site URL and the end you will get access_token on site URL. Copy the access token and paste in extension.