Within a specific collection, next to each of the samples: Emerging, Proficient and Advanced, you will see the option to 'Upload artifacts'. When you click 'Upload artifacts' a modal will pop up providing several options for where you can upload work from: 'Upload from computer', 'Import from Google Drive' or 'Import from other services'.
Once you select your files to upload, you will see the following modal:
You can use the trashcan to remove the file, click '+ Add Another File' to add another file, or you can 'Save & Score Later' or 'Save & Score Now'.
Learn more about scoring work: Scoring artifacts