Log in to Business Central as the Global Administrator for your Office 365 account using your usual URL.
Edit the URL and add the word "/admin" to the end of the link like below:
This will open the Business Central Admin Center for you Business Central Environment in the cloud.
NOTE: you have to be the Global Administrator of Office 365 to see this.
You will then be able to see and manage your current environments.
To create a new environment, use the "+New" icon.
In the Creative Environment window, you can specify your environment name e.g. Sandbox.
Select its type "Sandbox" and you can select the "Copy Environment" field if you want to copy your existing production environment in your Sandbox.
If you would like to work with a dummy environment, leave it blank.
Click "create" and voila! A new sandbox environment will be created and you will be supplied with a URL to use in order to access it.
There are a few different ways to swap between the environment but the way we like to do it is to simply 'bookmark' the environment at the top of your browser page beside the likely bookmark of your live Business Central.
Another way to do it, is to simply add "/sandbox" on the end of your Business Central URL (see the image below).
When you are redirected, you will be able to notice you are in your sandbox environment because of the "Sandbox" icon on the top right.
To swap back to your live site, remove the "/sandbox".