The number one search on the Critical Path website is for a Critical Path and Float worksheet.  Though you should be using software to calculate a critical path, if it is mission critical, it is important to understand the concept for the PMP exam. Rather then go into the specifics on how to calculate the critical path and float in this post, I'll merely say a free worksheet template  and PowerPoint presentation are available and you can download them at any time. (see links below)

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Remember the Critical Path tells you the activities that can not slip a day without increasing the total duration of the project or moving the project completion date. It is the longest path of logically related activities through the network which cannot slip without impacting the total project duration, termed zero float.