Three Basic Steps to Consider

Planning for capacity breaks down into three simple elements: determining your targets (KPI/Service Levels), understanding current capacity, and of course planning ahead.

Targets
In this step, your business should break down work into categories with a full list of internal targets and hopefully produce documentation quantifying users expectations of how work gets completed. (if this is not done in your organisation far more preparatory work will be required)

Businesses usually choose to organise workloads by either who is doing the work, the type of work performed, or the work process.

A "service level agreement" lays out the acceptable parameters between the provider and the consumer and is often the go-to for 'targets'.

Understanding Current Capacity
Organisations should always know the amount of staff they have employed. However, this information is often kept within HR or Finance and often their particulars of measurements do not match the need for operational capacity clarity.

Always ensure the amount of time each person spends somewhere/doing is well understood by all involved in capacity planning.

Planning Ahead
This is the easy one! Once you have put together your capacity plan from a) understanding your targets and b) understanding current capacity you will have created an output usually in the form of FTE and financials.

However, it is exceedingly important that you don't just leave it there. It is very important to organise Capacity Plan reviews, have an ongoing strategy to review 'current capacity methodology and ensure you are aligned with the strategic corporate objectives for your targets/KPI. If this is met in the most appropriate method for your organisation you will now be able to comfortably maintain your capacity plan.