Assign tasks from your workflow plan to each position. Keep the tasks specific enough that they fall underneath the job description of only one position. For example, a project requiring both a content editor and copy editor should list "edit content for factual accuracy" and "edit content for grammar and clarity" as tasks rather than the more general "edit content."
Add together the total amount of time required to complete each task. Use the projections from your workflow plan. For example, if "edit content for factual accuracy" requires 14 hours for each piece and the content editor's other tasks require 80 hours each week then a project producing three pieces of content per week would require 122 hours (42 hours plus 80 hours) of work from content editors.
Divide the total hours of work required weekly for the position by the length of a full-time work week in your organization. For example, if a full-time employee in your organization works 40 hours per week, divide 122 hours by 40 hours per week per employee to equal three full-time employees' worth of work plus two extra hours of work.
Repeat for each position. Check that all tasks required in the project are accounted for in the total hours of work.
Adjust the staffing model as the project begins. Continue to adjust the time budgeted to each task to reflect observed performance.