Upload the WP Post Thumbnail plugin to your wp-content/plugins/ directory then activate it within the WP Admin panel under Plugins.

Go to the WP-Post-Thumbnail Settings page in the WP Admin under the Settings section. From there you will see some different options you can set. Under common settings, you can set to your own liking.

Under custom thumbnail settings, you need to set the image dimensions and custom key/field for that individual thumbnail/image. In the majority of our themes we use the custom field "thumbnail" for thumbnail preview images, and "featured_thumbnail" for the featured content area. For the image dimensions check the theme's FAQ page. Depending on the theme, it may use additional custom fields or dimensions not mentioned here, refer to the FAQ page for the theme for more information.

After setting up the WP Post Thumbnail plugin it is now time to test to make sure everything is working properly. Edit or add a post and upload an image to the post's media gallery. Once added to the media gallery, scroll down to the WP Post Thumbnail section and click the "Reload" button to refresh the media gallery items for that post. Select the image you want to use and crop it to your liking. After you are done click the "Save Thumbnail" button to save the thumbnail to the custom field.

Some bugs I have noticed while working with the WP Post Thumbnail can often easily be fixed by deactivating and reactivating the plugin on the Plugins page. If that does not work for you, try deleting and re-uploading the plugin. If you are still unable to successfully get it to work, some other plugins might be conflicting causing it to not work.