I don't know why you seem to have so much trouble with simple things.
The page I pointed you to contains the following:
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Option 1: Use your current phone number in Google Voice
You'll be charged a $20 fee to move your number to Google Voice from most cell phone service companies, like Verizon or AT&T. But, moving a number to Google Voice after canceling Project Fi is free.
Corporate, landline, Internet-based (VoIP), and some other numbers can't be transferred.
Don't cancel your phone plan before you start the transfer. To verify the transfer, we'll call your phone with a code. After the port is finished, you can cancel your phone plan.
1. On your computer, open Google Voice.
2. If you haven't used Google Voice on your account before, set up a new Google Voice account. You'll be prompted to pick a new number, but your transferred number will replace it.
3. At the top left, click Menu Menu and then Legacy Google Voice. Google Voice will look different, but you're in the right place.
4. At the top right, click Settings Settings and then Settings.
5. Click the "Phones" tab.
6. Next to your current number, click Change / Port.
7. Select I want to use my mobile number. Follow the onscreen instructions to set up your new number and pay.
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Step 2 is not applicable to you as you already have an account and a GV number. You merely wish to change that number to the one you are porting in.
After step 5 above you should see your version of the attached screenshot.
After you have paid the $20 fee your GV number will be the one you ported in.
Your GV number is a different thing from the forwarding number(s).