Here are the steps:

1. Login to Google analytics account

2. Select your property. Go to Admin tab.

3. Inside View section, Click on Goals link.

4. Create a new goal. Select custom option.

set-new-custom-goal5. Set goal type as a destination.

set-goal-type-as-destination

6. Set the destination URL . ( You will set the same URL which you have set as redirect URL in ConvertPlug after form submission ).

To set redirect after form submission in ConvertPlug, check this link – Redirecting user after successful submission

For more details on setting goals for google analytics conversions, check this link – Setting up goals in Google Analytics