Open the Google Messages SMS application on your smartphone and write a message as you normally would. Once you've finished your missive, press and hold your finger on the send button.
If you see a window indicating Urgent and Subject (image on the left), you still have the old sending interface dedicated to MMS. You will therefore have to wait a few more days, or even weeks, before you can program the sending of SMS.
When the Send Schedule window appears, you will be able to choose one of the predefined suggestions, or Select a custom date and time.
Once you have entered each of these parameters, press Next and confirm the programming of your sending by pressing Save. For your sending to face itself correctly, your smartphone will obviously need to be switched on, but above all it will need to be connected to Wi-Fi or the mobile data network.