To start, you may choose your time horizon and the display -or not- of time off by selecting the corresponding option in the visual settings:
Capacity planning may then start with the overall planning of projects workload (as it is often what has been agreed with a customer, in the framework of a quotation for eg.).
Accordingly, the Project Capacity Planning view lists every projects currently active in your organization. It allows you to enter in the line "As Planned" the number of days you estimated for each project in the course of the coming months.
Just below the As Planned line, you will notice that the As per Staffing line is greyed out. It contains the number of days that you may have planned in the Users Capacity view ; you can detail the overall number by clicking on the downward pointing arrow, so to see each user's planned number of days.
The line Ratio compares thus the number of projects planned days with the total of users days, showing a color code for "over" and "under-staffed projects"
Last, note that you may use filters to only select some of the desired projects.