If there are no results on Libgen, you will need to use Sci-Hub.
1. First, you should find the place from where you could legally download the paper you are interested in if you were an academic institution with too much money. You can usually do this by searching the paper's title.
2. Next, visit sci-hub. Again, DuckDuckGo for sci-hub if the domain you usually use suddenly goes dark.
3. Paste the URL you found in step 1 into the search bar and click "open". (First picture.)
4. You might need to fill in a captcha.
5. It will try to find a proxy from inside an university that can download it. Wait, this can be really-really slow. (Second picture.)
6. If it succeeds, you will get the PDF (Third picture). If it fails, it might show you the website that you pasted in and you will have to navigate to the place where you can download the article. In the bottom there will be a grey bar, pressing the button (like the play button on radios) will give you a new proxy.