OPTION 1: Using an external software like Tally
Go to services-> check the service Id you wish to raise invoice for->click Edit
You will be taken to another page.
In that page, kindly click on Financials tab
Click on Add an invoice and enter the necessary details of the invoice as generated in Tally.
We are working on the integration with Tally to do this more seamlessly.
OPTION 2: Using Mulberry
Firstly, Kindly go to Admin => Settings and check the "Use Mulberry" option.
Now, the "Add an invoice" in the financials tab in services will change to "Raise an invoice".
For further understanding on how to raise an invoice, Kindly go through the Blog.
https://blog.papilio.co.in/2011/11/introducing-mulberry.html