For scripting and scheduling in general, start here:
https://winscp.net/eng/docs/guide_automation
https://winscp.net/eng/docs/guide_schedule

To select the yesterdays file only:
If you can rely on a relative time between creation of the log and run of the script, you can do with simple file mask with time interval constraint, like:

See https://winscp.net/eng/docs/file_mask

For other (more exact, but also more complicated) solutions see:
https://winscp.net/eng/docs/script_download_most_recent_file
https://winscp.net/eng/docs/script_download_timestamped_filename

Hey guys, everyday I have a server that has a log file made every day, the log file has a name such as log_20130702 and log_20130703 etc etc. What I want to do is every morning take the log file from the previous day (I.E. if today is July 3rd, take the log from July 2) and throw it in a FTP server. I would want this to be done automatically. How would I accomplish this?