There are many different solutions available for creating screencasts. We're going to use Screencastify (www.screencastify.com) because it's free, it's easy to use, and it works as a Chrome browser extension and with Chromebooks.
Install Screencastify by logging into your Google account in the Chrome browser, visiting www.screencastify.com and clicking the Install Now button. This will install Screencastify as a Chrome extension in your Chrome browser.
Alternately, and as you will see in many Screencastify tutorial videos, you can install the Screencastify extension via the Chrome Web Store.
One great way to learn how to create a screencast using Screencastify is to just dive in and try it! No pressure, no wrong answers, just try, learn, try again, learn more, grow, you get the idea.
Peer exploration is another great approach...set-up side-by-side with a colleague and start making screencasts. Two heads may well be better than one, with you and your colleague able to observe, discuss, coach each other, etc.
Tutorials can also be helpful...here are some solid ones we've found (though no shortage of others online).
Alex Reyes or El Paso, TX hosts an online site called Digital Maestro containing a "text and image" tutorial titled "Setting Up the Screencastify Extension" which can be helpful when you are first setting up Screencastify.
If you'd like a video approach to set-up, here's a popular from YouTube by Chris Betcher, "Introducing Screencastify".
Kathy Schrock has a section of her "Kathy Schrock's Guide to Everything" site dedicated to screencasting, notable especially for her focus on screencasting and pedagogy. Lot's of ideas and resources here!
Finally, do feel free to search and find screencasting resources that work for you. No shortage of tutorial videos and pages out there!