I would like to know how to sit during the Friday sermon.

There is no specific sitting posture for listening to the Friday sermon. A Muslim should observe etiquette and moderation, so as not to harm those around him, nor sit in a way that exposes his awrah (private parts), nor sit in a manner that induces laziness or drowsiness. Some scholars, like the Shafiis, disliked the ihtiba’ (buttock-sitting) posture because it can lead to sleep, citing a hadith that many scholars have deemed weak. Furthermore, some Companions and Tabiin, as well as the Imam, would sit in the ihtiba’ posture while the sermon was being delivered. It is preferable to avoid ihtiba’ because it may cause sleep, but there is no harm in it if it does not lead to that.