Aushutosh,
Normally this is configured by the functional persons. But its always good to understand how this is done and how the same works. Below are the steps (on the high level)
1. On the transaction NACE, choose the the correct application. For invoice its V3.
2. Select V3 and hit o/p types
3. You will need to create a new entry.
4. For the new o/p type that you will be creating, you need to assign the form and the driver program that will be called when the o/p type gets processed. This you do by selecting the o/p type and selecting "processing routines" on the left side. You will need to specify the Program, Form Routine and the Form.
4. Next you will need to specify the partner function, again available on the left side.
5. When you double click on the o/p type, you will see the field called access sequence. Here you need to specify your (condition document type and storage location) and the maintain the same.
So once the condition in step 5 is met, the driver program will be called and pass the data to your form.
Hope this helps.
Thanks,
Vikram.M