1. First of all,search for your required Google Chrome extension like this. Let's take Adblock Plus as an example.
2. If you got the download link then download it via a IDM not by Chrome itself.
3. Now, you have the downloaded extension. Copy this chrome://extensions/ and paste in address bar. You will see some or no extensions.
4. Drag the downloaded extension and drop in Google Chrome's extension page to install it.
6. Wallah!!! :) You will see that extension is installed.
If you haven't read the 1st post then click this - How to make Google Chrome fast | With Pictures
Hope you will like this post as it involved a lot of hard work and patience to write on such an obnoxious topic. By the way, Thanks for reading.. :)