Go to the invoice you want to write-off and click on Add Payment
To write-off the invoice as bad debt:
Fill out the Date.
For the Deposited To choose the Bad Debt expense account. If you don't have the account, learn how to create an account by viewing the article Setting Up Accounts.
Enter the Amount.
If it was in a foreign currency, make sure to Exchange Payment.
If so desired, enter a Memo explaining the reason for non-payment.
Click on Add Payment.