What is a critical path? It is a method of looking at all of the activities in the Gantt chart, considering the timing and dependent relationships of each, and calculating the longest path from start to completion of the project. This process determines which steps are "critical" from those that can be delayed (said to have "total float") without risking making the project late.
To define the critical path, use lines to connect a task to another if they are dependent on each other. If the completion of one task will trigger the beginning of another, draw a line from the end of the first to the beginning of the second with an arrow pointing to the second to show that it can begin after the first is finished.