How to create a sandbox instance for testing
It is absolutely best practice to never make major changes or test new software in your main production instance. If something goes wrong, you can have a big mess on your hands. Instead, best practice is to make a Sandbox that you can use for testing.
What are the different types of sandboxes?
Salesforce provides all nonprofits with a choice of 2 different sandboxes for no additional cost. Either Developer or Developer Pro. The key difference between these two are how much data can be stored.
In both cases, NONE OF YOUR DATA IS COPIED TO THE SANDBOX. This is critical. When you create a Sandbox, you are only getting your setup information - fields, configuration, etc. No records. No users. You will have to create test records on all objects first before you can test anything else.
If you want a sandbox that includes your data, you will need to speak with your Salesforce Account Executive about purchasing a Full or Partial Sandbox at an additional cost.
To view your sandboxes, click Setup and type Sandbox as shown, then click on the result.
Here you can see the Sandboxes you have available. Nonprofits typically get 6 Developer and 1 Developer Pro.
Under the Sandbox History tab, you can see all the times the sandbox was refreshed.
Logging in to a Sandbox
You can log in to a sandbox by clicking on the Login link on the sandbox list. Your login for the sandbox will be the same as your current instance except your username will have the sandbox name appended on the end.
If you want, you can go to https://test.salesforce.com to login to a sandbox instead of going to your organization's sandbox screen. Just make sure you use the right sandbox username as above.