ArcMap provides a variety of tools for creating linear features. A route is simply a linear feature with measures, so the techniques for creating routes are similar to the techniques for creating other linear features.
Selecting the features
When you create routes from existing lines, you must select the line or lines whose geometry you want to copy to create the route. The selected features can be from one or more of the linear feature layers already in your map or from any supported data format.
Learn more about supported data formats
You can use any method available to select the linear features. For example, you can use the Edit tool 
on the Editor toolbar, the Select By Rectangle tool 
on the Tools toolbar, or Select by Attributes from the Selection menu.
Setting the parameters
Once the linear features have been selected, use the Make Route dialog box to set the parameters for how your new route will be created.
The first parameter you set is the start point. This is where the route measure values will begin. There are two ways to set the start point. First, you can click a point on the map. When you choose this option, you will note that as you move your pointer around the map, the start point automatically snaps to one of the selected feature's endpoints. Second, you can use a coordinate priority of lower left, lower right, upper left, or upper right. These coordinate priorities are determined using the minimum bounding rectangle around the selected set of linear features. The endpoint from the selected features that is closest to the chosen coordinate priority will be where the measures start.
The next parameter you set indicates how the measure values are to be obtained. When you choose the Geometric Length option, it is important to realize that the geometric length of each selected feature is determined in the units of the target layer's coordinate system, not the feature's native coordinate system. This is because within a single data frame, features with different coordinate systems can be projected on the fly.
The Measure Field option is only available when all the selected linear features are from the same layer. Measure values on the new route are accumulated using the values in this field.
Learn more about measurements and route data
Use the From/To option when you know the start and end measure values for the new route. All measure values between the start and end will be interpolated for you.
Multiplying measures is useful when you want to convert between measure units. For example, your route data is stored in a feature class with coordinate system units in feet. You want the measures on your routes to be in miles. You would multiply your measures by 0.0001893994 to convert from feet to miles. The measure multiplication factor is applied before the selected lines are merged to create a route.
The Start measure option is useful when you want your new route to have a start measure other than 0.
The last parameter you set indicates whether you want the measures to be continuous on disjointed routes. Note that if you choose to have noncontinuous measures, the gap distance is calculated using the units of the target layer's coordinate system. This might cause unexpected measure values; for example, you will get unexpected measure values if the target layer's coordinate system units are meters and you are using a field that stores mileage values to accumulate the measure. Note that this check box is not available when using the From/To option to set the measures.
Setting the attributes
Once the new route has been created in the target feature class, the selected set of input linear features will become unselected and the new route will become selected. This is so you can set the attributes, such as the route identifier.
Complex routes
The process of creating a complex route is very similar to that of creating a simple route. The only real difference is that you must build a complex route in pieces. Once the pieces are created, they can be merged.
The workflow shown in the task for creating a complex (looping) route is not the only one. With your knowledge of editing in ArcMap, you might choose a workflow that is more appropriate for your organization.
Take note that in creating a complex (looping) route in the task, care has been taken to set the measures appropriately for each of the two halves of the route that are eventually merged. If this is not possible in your situation, you can still merge the pieces. Then at a later time, the measure values can be reset.