Click the Start circle and click Caret. If you don't see it, click the ^ icon at the top of the opened screen and it will list icons for all the apps available. Scroll through and find Caret. Click the Caret icon to run.
The following page is from CircuitPython Made Easy on Circuit Playground Express - the Download file link is to this code on GitHub.
When you click the Download file link, Chrome OS will open a File dialog asking where to save the file. You can save it on the Chromebook flash drive My Files -> Downloads or your own thumb drive.
Nearly all Python/CircuitPython files end in the file extension .py. When saving the downloaded file, you can select a meaningful name like servo.py or you can name it the same as you will name it on your board: code.py.
Run Caret by pressing the Chrome Start circle and selecting Caret.
You'll get the following screen:
You can start typing code now.
If you have existing code or if you downloaded code, you can open that file now. Select File -> Open and you get a File dialog box where you can select your code file (usually ends in .py).
It is possible you want to edit the code already on your board. Navigate to the CIRCUITPY drive and click code.py to open.
Edit away. Caret will color code your code as it knows it is Python (see lower right of screen).
Save your work via File -> Save File As... and name your code appropriately. We suggest naming the file you intend to immediately use code.py as that is what CircuitPython will expect.
You probably want to save the code also on Google Drive or your own flash drive (if you do not own the Chromebook to ensure you have a copy). The code will be saved on the flash of your microcontroller board but that is not the safest place, copies are good!
Your code is edited and saved to a flash disk. You now want to put it on your device and run it.
Run the Files app. Be sure your find your code. Your program should be named code.py to be able to be found by CircuitPython (not just anyfile.py, actually named code.py).
Plug your board (like an Adafruit Circuit Playground Express or other CircuitPython compatible board) into your ChromeBook via a known good USB A to micro-B cable. In the Files app, a new drive appears named CIRCUITPY. If you get a CPLAYBOOT (or other BOOT) drive or no drive, you probably need to install CircuitPython on the board. Follow this guide to do the install.
Now drag your CircuitPython source file (code.py) to the board. The code should immediately run.