It's nice to see you here in the Community, @littletwoloves.
I'd like to let you know that you're already on the right track. I want to commend you for doing a great job in this process.
Creating a bonus check is the best way to record a lump-sum contribution for the 401k retirement plan in QuickBooks Desktop. This because the process covers the regular deductions of your employee which can be applied to the bonus amount.
To learn more about the types of bonus paycheck that you can create and on how to pay your liabilities in the system, you may consider checking these resources attached: