1. Go to Marketing > SMS > Compose.
2. Pick the From number.
3. Click the + in To and choose one or more recipient groups in the Select Contacts window, then DONE. Optionally add groups under Exclude.
4. If you picked an "on X day" group, set the Trigger Date.
5. Write the text in Message Content (or Use Template) and insert @ Shortkeys if needed. Watch the Characters Remaining / Message(s) / Cost counter.
6. Under Campaign Type keep the Schedule tab, choose a Schedule Date & Time in the future and click Next Step.
7. Review the Review & Confirm window (Recipients, Total Cost, Balance After, Groups, Message, Scheduled) and click CONFIRM.

You will see "Successfuly saved for N contacts"; the messages are queued and sent by the background job at the scheduled time. Track them under History > Scheduled / Sending / Sent and Group Messages.