To install extensions on Ungoogled Chromium, you need to download .CRX extension files, in this tutorial you will learn how to download these files and manually install extensions on Ungoogled Chromium and other chromium based browsers.
First you need to visit this link on chrome without the brackets ( chrome://flags/#extension-mime-request-handling )
Or just visit chrome://flags and search for “mime” then change the value of Handling of extension MIME type requests to “always prompt for install”

Then relaunch chrome when prompted

Then from chrome (or any other chromium based browser) go to extensions by clicking on the 3 dots at the top right corner, then extensions, then manage extensions.

Enable developer mode if it’s not already enabled

Using Chromium web store:
Visit Chromium web store on https://github.com/NeverDecaf/chromium-web-store/releases and click on the .CRX file

Then click on add extension

Then navigate to any chrome extension you would like to add on chrome web store such as ublock origin https://chromewebstore.google.com/detail/ublock-origin/cjpalhdlnbpafiamejdnhcphjbkeiagm and click on add to chrome

Then click on add extension

Done.
Using CRX extractor websites:
visit one of these websites:
https://www.ungoogledextensions.com
copy and paste the link to any extension you want on chrome web store

Then click on add extension when prompted

Done.
Using CRX-Download by tonystark93 on GitHub
Then copy the link of this extension https://chromewebstore.google.com/detail/crx-extractordownloader/ajkhmmldknmfjnmeedkbkkojgobmljda (CRX Extractor/Downloader by thebyteseffect.com) and paste it on one of these sites:
https://www.ungoogledextensions.com

Then click on add extension when prompted

Then navigate to any chrome extensions you would like to add on chrome web store such as ublock origin https://chromewebstore.google.com/detail/ublock-origin/cjpalhdlnbpafiamejdnhcphjbkeiagm
click on extensions button then click on the button for the extension we have just installed then download as CRX and choose where to save the file


Then from chrome (or any other chromium based browser) go to extensions by clicking on the 3 dots at the top right corner, then extensions, then manage extensions, and make sure developer mode is on.
Then drag and drop the extension .crx file you have just downloaded anywhere on chrome extensions page

And confirm adding the extension when prompted

Then repeat this process with any other extension you would like to install.