By 'Manually' I mean an installation without defining ftp settings on Opencart 2 settings page. Also, everything described below is applicable to extensions created by me. I cannot say anything about extensions of other developers but maybe these steps work for them too.
1. You should unpack the zipped archive to any folder on your computer. The archive contains:
- 'upload' directory with directories and php files
- 'xml' directory with xml files
- any other files
2. Upload all files from the 'Upload' directory to your store. Directories should match directories in your store. DO NOT upload the 'Upload' directory itself.
3. Install all xml files from the xml directory on the 'Extension Installer' page as regular modifications. When you upload pure XML files ftp access is not required by opencart.
4. Go to the 'Modifications' page and click on a 'Refresh' button there. If anything goes wrong, here is an article describing how to recover the store after uploading any modification: How to Recover Store After Installing a Wrong Modification.
5. Go to the 'Users / User Groups' page and grant all the required access to new pages to a user group you belong to.
Performing these actions is equal to uploading a zipped archive. Next, you can follow installation instructions from the readme file enclosed in the extension archive.