The prefix used to address Carl will depend on how it is set up in your server. In the NEISVoid server we use @Carl-bot to make it explicit that we are addressing Carl, but on other servers this may be !, ?, or something else.
To get the JSON of an embed, run @Carl-bot embedsource $message-id $channel. If the output is more than 2000 characters it will be uploaded to a paste site and you'll be given a link, otherwise it will be returned in the channel where you invoked the command. Wherever it is, you can copy the JSON and paste it into the textbox below. Assuming you hve enabled javascript in your browser, this will populate the editor.
Once the editor has been populated, you can edit it using the various form fields, and the JSON textbox should update in real time. Once the editor has all the information you want, you can copy the JSON from the textbox, and use it with @Carl-bot ecembed $message-id $channel $json. If this is longer than you are able to post (2000 characters unless you have Nitro, otherwise 4000 characters), you will need to upload the JSON to Pastebin and run @Carl-bot ecembed $message-id $channel $link-to-paste.