You can upload a Google Doc template that you've created under the Templates tab in the right main menu. To upload the template, click on the Select Template option and choose the one you would like to upload. Your two most frequently used templates will also appear to the left of the Other option for easy access.
Once uploaded, you'll want to edit the template so it will be customized for your specific recipient. To do so, click the Edit button symbolized by a Pencil in the top right corner above the document. A window will then appear and you can edit the template just as you would in Google Docs. Click the Close button in the top right to close and save the document for sending.
If you would like to upload another template or document, scroll below the already uploaded doc and repeat the process; choosing from the Upload Rectangle, the Templates option, or the Other option. Once your document is uploaded, you can rename it or add a deal value to it (recommended if it's a sales document). You can then add your recipient(s).