Keep in mind that no work is ever actually done at the campaign level, so things like time & expense entry, tasks, assignments, files, diary conversations, etc, are all tracked at the project level. In light of this, some users have created a "campaign project" where this type of data can be used as a repository for the campaign.
Projects can be added at any time, even closed projects can be linked to a campaign.
Ways to link a project
There are 3 methods for adding projects to a campaign (see screenshot examples below).
Link existing projects to a new campaign
Click the "Projects and segments" tab located along the left side to search. To add specific projects that already exist click the "Link projects" option under the More option.
After adding projects you may want to further divide them into "segments". This is another grouping for budget purposes and is often used in relation to the seasons of the campaign.
NOTE: The above screenshot shows the Use multiple segments options checked, with projects nested under each segment.
Create a new project from an existing template
From the bottom-right corner, project templates area, drag a template to the left. This will auto-create a new project using the selected template. The client for the new project will be based on the client specified in the Campaign setup screen.
Link the existing campaign from the project side
From within an individual project go to >Setup>General>Campaign: search for & select the appropriate campaign. If the campaign is set up with segments, this field will be available once the campaign is selected. You must select a segment.