Once you are logged in, you will see the files. Under the files, you will find the uploader. Upload your files and use the "Copy to clipboard" links.
Have your "Uploads" page open and you will see a list of files that are available. Beside every file is a link that says "(Copy to clipboard)". Click that link and go to the article you are writing in Ghost. Select or type and then select the words that are to become your download link and press "CTRL-V" to paste the link that is in your clipboard.