how and when do you charge the mouse on the new M1 computers?

Recharge it whenever you wish.

Click the Bluetooth icon in the menu bar to see the battery charge status of your mouse and keyboard.

If the Bluetooth icon is not in the menu bar, click System Preferences > Bluetooth and check the box to Show Bluetooth in the menu bar.

Recharge the built-in battery in your keyboard or mouse - Apple Support:

https://support.apple.com/en-us/HT205160

You will get a notification when battery in mouse needs charging however you should have another mouse handy because the mouse cannot be used when charging.