To create twitter application for using it with Tweet Old Post wordpress plugin follow following steps,
1 – Login to Twitter Developers Site with your twitter credentials
2 – Hover on your username on top right hand side and select “My Applications”
Hover on username to get to applications
3 – Click on create new application
Click on Create Application
4 – Create the new application, accept the rules enter the capture and create
Enter all necessary details
5 – change the access permission for your application.
5a – Move to settings tab of your application
Move to settings tab
5b – Change the access to read and write
Change access level to read and write
5c – Update the settings
Access level should change to read and write in details tab
6 – Scroll at the bottom of your application details page and Create access token by clicking on create access token button
Click on create token in details tab
It should look like this after access token is created
7 – In the settings tab, in callback URL, enter the url of your home page. For e.g. https://www.ajaymatharu.com/index.php (just a URL it will not go to that URL)
8 – Copy the consumer key and consumer secret and paste this on tweet old post settings page and update settings.
9 – De-authorize and Re-authorize again for it to work (Thanks to DiTesco)
Feel free to drop a comment or mail me the details if something does not works.
Update:
Added a new step 7 which is the solution to 401 status code in log file. It is tested and works.
Update
Added a new step 9 for people who were not able to make it work