Start with the workload, not the server.

Describe the job.

List the tasks the application performs, the information it reads and writes, and the services it depends on. Separate interactive requests from scheduled work.

Understand the busy period.

Think about when demand arrives. A short peak and a steady increase call for different capacity decisions. Start with measurements you can repeat and compare.

Choose a useful margin.

Leave room for expected variation, maintenance and small changes. Record why that margin is reasonable instead of treating it as an unexplained number.

Review as the system changes.

A useful capacity plan is a living description of a workload. Revisit it when the application, data set or pattern of use changes.