Almost all project schedules are built using a Work Breakdown Structure (WBS). When you're building a complex schedule, you don't sit down and identify fall of the activities starting with the first one and ending with the lastYou probably first enter your high-level work, then come back later and start filling in the detailed work. This is basically the WBS technique.
If the project manager is the only person creating the schedule, it's very likely that the WBS will be created at the same time the work is entered into your project management scheduling tool. However, in many cases the project manager doesn't know enough to lay out the entire schedule from scratch. In this case, a small group of people may be required to complete the WBS.
When you're creating a WBS in a group setting, it may be best to use Post It notes and a blank wall to create the first draft of their Work Breakdown Structure.
This technique is very easy. After you get the appropriate people into the same room (project team members and clients who have the expertise to build the WBS), you start to build it. You can start off by writing the names of the major deliverables on Post IT notes, one deliverable per sheet. Make sure the attendees agree on the major deliverables to begin with. If any of the deliverables are very large, you can create new notes that describe the deliverable at the lower level of individual work products. These work products are arranged under the higher-level deliverable.
The deliverable needs to be identified at a level low enough that you understand what it takes to build it. In general, two levels should be enough to identify your deliverables. One level is typical.
Next, for each deliverable, describe the activities that must take place to complete it. Each activity goes on a separate note. These activities are arranged under the specific deliverable they refer to. If you have a sense for the order that the activities need to be completed, you can arrange the notes sequentially. However, this isn't vital at this point. The important thing is to identify all the work.
Look at the activities that are required to build each deliverable (or work product) and estimate the work associated with each activity. If the effort associated with an activity is larger than 80 hours (or whatever high-level threshold you set), you would break that activity into a set of smaller activities. Each of these activities is represented by new Post It notes under the higher-level activity (which now becomes a summary activity).
Continue with this process until the work required to complete all of the deliverables are defined, as best you know at that point. Some simple deliverables may take one or two levels of work breakdown. Others may take three, four, or more.
The advantage of this approach is that your team can actually see the work and they can help ensure all the work is identified to complete the project. The Post It notes also give you the ability to easily move things around. If you add an activity and then decide to remove it, you just pick up the sticky sheet. Likewise, if a deliverable or group of activities is in the wrong place, you just move the sticky sheets to where they need to be.
When you're all done, you can enter the detailed work activities into your workplan management tool. Remember that the WBS is not the same as the schedule. The WBS is only the technique you use to understand the work at a low level. When the WBS is completed you can continue to build your schedule by sequencing the activities, assigning resources, estimating costs and duration, etc.