We recommend the following process:
1. Upload images you want to schedule to the Content Manager.
2. Download a template and open it in your favorite spreadsheet program. Remove the existing posts from there.
3. Paste image names from Content Manager to the link column.
4. Set a title, subreddit, and post_at for every row.
After you upload your spreadsheet, you can fill it further according to the errors (for example, add flairs where they're required).
When filling columns that are the same or similar for every row (post_at, for example), you can use autofill to speed up the process.
It may take a bit longer to create your spreadsheet for the first time, but after you have it, reusing it is speedy. You only have to edit the link and title columns.