1. On Compose, fill in From, To (and Exclude / Trigger Date if needed) and the message as usual.
2. Under Campaign Type switch to the Recurring tab.
3. Set the Start Date and Preferred Send Time.
4. Choose the Repeat Type — Weekly (then pick days under Repeat on these days) or Monthly (then either Select specific days 1-31, or On the specific weekday: First/Second/Third/Fourth/Last + a weekday or "Day").
5. Set Repeat Every N weeks/months.
6. Choose the Ending Condition: Never (runs until manually cancelled) or Specific Date.
7. Click Next Step, review the summary (Recipients, Est. Cost/Run, Balance After, Schedule, First Send, Next Send, Ending) and CONFIRM.

The campaign then appears under Marketing > SMS > Recurring. Recipients are recalculated at every run, so the cost per run can change as your groups change.