This is just an opportunity to point people to Apple’s excellent help page on this topic, with instructions and screenshots. Go here for more https://support.apple.com/en-gb/HT208721
Shift+Command+5 brings up a resizable window (adjust to select an area you want to record, or click the ‘whole screen’ option). You can tweak the settings to include the inbuilt mic if you want your video to have spoken instructions. To end the recording you need to press Shift+Command+5 again at which point you can edit out the ending (the bit of you moving the mouse to press ‘stop’) from the recording, and you can shorten it at either end.
Give it about 3 seconds after you press record before speaking as there seems to be a slight delay and I’ve found it generally misses off the first few words. Recommend doing a test 10 sec recording first.
Once you’ve completed the recording a small pop up version appears at the bottom right of the screen (on my system) and clicking it brings up the video with a panel at the top for editing. Click the button on the left of Done to shorten it (first pic below), and use the yellow drag bars (2nd pic below) to shorten it. You can click anywhere in the ‘tape strip’ and press play to see how your new ending / beginning changes things.
Here’s one I made earlier, which relates to Make Twitter Useable Again