In general, what you need to do is to find the option to export your contacts to a file that can then be imported into your Google account. I'm not familiar with Windows Phone, but check your Contacts app and see if there is an option to export your contacts. If so, ideally you should export them into a .csv file. You could then move that file to a computer, go to Google Contacts on any web browser, and then import that .csv file into your contacts.
If you sync your Windows Phone contacts with Outlook, it may be easier to export the contacts from Outlook.
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