On the initial “Site Login” screen that opens when WinSCP is launched, fill in the below details:

File protocol: Amazon S3
Host name: s3.amazonaws.com
Access key ID: <access-key>
Secret access key: <secret-access-key>

Then click on Advanced, navigate to Environments > Directories and fill in the remote directory (which is the S3 bucket name).

Remote directory: /<bucket-name>