Step 1: If your video is under 7MB, the file is attached to the notification email sent to you when you exported the project. Download it to your phone and go straight to Step 3!
If your file is over 7MB, click the link in the email and upload the file to a service such as Dropbox. You can easily do this from the download page.
If you need help find this download page check out How To Access and Download Your Exported Video
Step 2: Now that Dropbox has your file, go to the Dropbox app on your phone and download the file
Step 3: Upload it to Snapchat.