The easiest way to block websites is to use an extension for Google Chrome.

The most popular one is called Block Page for Windows:

https://chrome.google.com/webstore/detail/blocksite-block-websites/eiimnmioipafcokbfikbljfdeojpcgbh?hl=en

For Android:

https://play.google.com/store/apps/details?id=co.blocksite&gl=GB

The more complex way is to follow the steps on this page to block websites in Chrome.