You need to go through the available applications that your Smart TV has to see if there is one that will allow you to record the channel you are watching. I am going to assume it is a standard feature for all Smart TVs. Read through your manual perhaps it will have the information you are looking for.
As demonstrated by the videos for the Samsung and LG Smart TVs all you need is an external hard drive with a USB cable to record TV channels. I can't help you beyond that since I do not even own a Smart TV.