By default, QuickBooks will automatically assign numbers to your sales forms starting with 1001. If you want to use your own numbering format, we can simply customize the number to whatever you prefer. I'll be guiding you in doing the whole process.
Before diving in, we'll have to take note that the sequence you set here will be applied to all of your sales forms and the invoice numbers are will be printed for your customers.
Here's how to customize the number:
Click the Gear icon.
Select Account and Settings.
Go to the Sales, then click the Pencil icon in the Sales form content section.
Tick the checkbox next to Custom transaction numbers.
Click Save, then hit Done.
Then, we can now open an invoice or any sales form, then enter the custom number or letter sequence you want to use for your forms in the Invoice no. field.
Once you're done, we'll have to turn off the Custom transaction numbers feature to prevent accidental changes to your numbering. Turning this feature off only means that the Invoice no. field is hidden from your sales form.