To upload your docx file to Atticus, you'll navigate to your Home page and click the large 'Upload' button that has an arrow pointing up. Then you'll select the docx file from your computer, input whatever relevant title data you would like, and click the green 'Go Get Em Atticus' button.
If the docx file has been formatted in a way that's compatible for Atticus, the upload should be nice and smooth. And to make sure you're making your docx file compatible, we do have recommendations to follow for preparing your docx prior to that upload here: https://www.atticus.io/how-to-prepare-your-word-document-for-upload/