If you're not sure if you have a business account, I recommend you try to log in first. If you don't have an account, it will prompt you to create an account.
Once set up, you should see a screen similar to this
Click on your business settings (top right)
Look for Ad Accounts on the menu that flies out.
Click the Add New button, then Create a New Ad Account
Put in an Ad account name, probably just the same as your business name. Fill out the rest (if it isn't already)
Add people to also be admins on your page. Not a required step.
Your setup is now complete!
Next, click on Partners and then Assign Partner
On the next screen, select the link Connect your ad account using your partner's business ID instead.
Under Role, select Ad account admin
Add in our Business ID: 629506900551583
If done correctly, you should see a page telling you you've succeeded.