Strava users can create segments to mark the most interesting parts of their cycling and running routes. Segments need to have three things: a start point, an end point, and a sequence of locations in between.
The first key to segment creation is that the data has to already exist on Strava exactly the way you want to create the segment. Each segment will based on the data it was originally created from.
To create a segment, begin by viewing an existing activity in Strava. Whatever activity you choose, that data will be the foundation of the segment, and affect the segment's accuracy and matching. For rides, click on the Actions button and then click on "Create Segment." For runs, click on the wrench button and then click on "Create Segment."
You'll then see the segment creation interface, where you can specify the start and end using the green and red sliders above the map. As you drag the sliders forward and back, the map updates the position of the green start marker and the red end marker. For single step adjustments, you can use the buttons labelled "back" and "forward" below the left and right ends of the slider.
For some helpful tips for creating good segments, please see this Knowledge Base article.
When you are happy with the start and end points, click "Next." Strava checks to see if a similar segment already exists.
After clicking "Next," again, you will have the opportunity to name the segment. Good names describe the segment in a way that will be recognizable by other people. It can be helpful to include the name of landmarks used for the start and end. Or use a familiar nickname known to locals. For example, "Alpe d'Huez" is a well known section of road leading up to the ski resort with the same name, and "Heartbreak Hill" is a segment of the Boston Marathon course.
Finally, decide if the segment is private. Private means that only you will see the segment, and only your results will be shown. Your results on the segment will not be visible to others when they view your activities that match the segment.