Please fill out the fields below so we can help you better. Note: you must provide your domain name to get help. Domain names for issued certificates are all made public in Certificate Transparency logs (e.g. https://crt.sh/?q=example.com), so withholding your domain name here does not increase secrecy, but only makes it harder for us to provide help.
To the question in the subject, you don’t. Let’s Encrypt has no web interface to upload and validate a CSR. To learn how Let’s Encrypt works, start here:
If you want a web interface to issue a cert you can look here, but that really isn’t the recommended way to use this service: