Once you have logged in, you will be able to see on the left hand side a list of all of the channels that you are a member of. Navigate to the channel that you wish to upload files to:

To upload documents, you will need to select the purple Upload Files button and once the dialog box appears, click on Select Files and then navigate to the files on your computer. You can add a number of files at one time by holding down Ctrl and selecting them one-by-one, or by holding Shift whilst selecting the first and last file in a list if they are consecutive or selecting the whole folder using Ctrl + A:

You also have the option to add an accompanying message and set a retention time.

You can also Drag and Drop files into the channel and when the purple dotted line disappears, the dialog box will open automatically with these files added:

The maximum amount of time files will ever be available for is 30 days and they will be wiped as standard once that time has elapsed. You can send up to 4GB of data in any one transfer, but please speak to Opus 2 if you wish to send something larger. Once you are ready, click Upload and the documents will then be added to the channel.