Wweeellll, first, you draw your part of the collab, then save the file, go to uploadfiles.io and upload the file you just downloaded, get the link and give it to the other person! Or, if your not starting the collab, when you receive, the link, you go there and download the file, then you go back to the editor and press the button next to the download button and click browse, when you see the file, click it!