Basically the answers you've already got. The pre-installed app, Simple SMS or QKSMS are your friends here. Try them and see what works best for you.
In an ideal world, you'd only need SMS in rare cases where you receive short codes (e.g. to register Signal on a new device).
Depending on your peers and also country culture, SMS might be the main communication tool. If you want people to use Signal instead, nice words and privacy education won't do (people do what's convenient and if nobody they chat with is on Signal, it doesn't make sense to them). Michael Bazzell has a good strategy: He tells people that he's best available on Signal and then makes sure to answer SMS with a few days delay and a sentence like "Sorry, I just rarely check on my SMS. If you need me to reply quickly, please use Signal instead". He also mentioned a friend that had Signal, but was barely available. He send him a Signal message mentioning he got tickets to the friend's favorite baseball team on short notice. The friend didn't check Signal in time and missed the game, but ever since he's eager to use Signal in case another great offer happens. For his family he sent a random family picture from the past to the Signal family group in order to trigger more interaction with Signal.
So depending on your situation, you might need to be creative. I wish you the best of luck, fortunately I got all my family and friends to use Signal after a while with little to no resistance, and we have a much better communication now in different group chats and even video calls.