Getting started depends on the file size. Is it over 400 meg? If not, you'll want to visit the Downloads section, navigate to the GamePro category, and click the Upload File button.
For the "Click to Upload Screenshots" section you'll want to upload a thumb of the issue cover. If I recall, something sized to around 500 pixels in height should do.
If you'd rather, you can make the file available somewhere online (Dropbox, Google Drive, archive.org, etc) and a RetroMags admin can upload the issue for you. We'll ensure you receive proper credit for the upload.
Thanks for your help with the preservation effort!