To add documents to a section, first select the "Add new" button at the top of the bundling screen.
The "Add new" menu will appear defaulting to the "Add document" tab. If you have connected your account to a document management service, use the respective tab to add documents from there.
Select "Choose file" or drag and drop documents into the prompt to select your documents for upload.
Once you have chosen your documents, they will appear in a list on the right. You can choose which section to add them to by using the section drop down, to the left of the "Add" button.
Once you are satisfied with your selected documents and which section they will be added to, click the "Add" button to upload them to your bundle.