Final answer:
To quickly make accounts subaccounts, you should click the diamond next to the account and arrange it under the parent account by dragging. This can be done even after accounts have been initially set up.
Step-by-step explanation:
To quickly make multiple accounts subaccounts of another account in the accounting software, the correct method among the options would be: Click on the diamond to the left of the account, drag it below the appropriate parent account, and release the mouse button. Then click on the diamond again, drag it to the right, and release the mouse button. This approach allows you to modify the structure of your chart of accounts without having to start from scratch or go through extensive reconfiguration processes.
Option 3 is incorrect because in most modern accounting systems, you are allowed to change an account to a subaccount after its creation. Option 2 may work in some systems, but it is not described as the standard, quick method. Option 4 could work depending on the software's features, yet it's a more contextual approach based on the system's user interface.