Short answer
A 24/7 plan starts with 168 hours of coverage a week. Multiply that by how many people each hour needs, convert it into paid shifts, then add buffers for leave, sickness, training, and handoffs.
Skip the buffer and the roster looks complete on paper, right up until the first person calls in sick and the gap has nowhere to go.
Define minimum staffing for every hour
Choose shift lengths and rotation rules
Include handoff overlap where context matters
Plan weekends and nights as first-class shifts
Add absence and leave coverage before publishing