Here are the steps that I do:

1. Right side New Button

2. Click on Invoice

3. Fill it out

4. On the bottom line click on "Customize"

5. Choose on of your custom invoice forms 

To create custom invoices for separate divisions:

1. Go to the Gear button on the top left

2. Click on Custom Form Styles

3. Hit New Style

4. Create Two Separate Invoice Templates (with different names)

5. Now when you create an invoice you have these options