Start by clicking on the Create a New Adventure button on the bottom left-hand side of the screen.
Select the Draw a Route option and begin by clicking on screen where you would like to start your route.
You will see you can now move your cursor to create the route and click to place the next point.
You can select the SnapTo option on the top of the screen (highlighted in the screenshot below), allowing the route to stick very easily to existing trails on the map.
The i button gives you more information on how to create your routes.
If you prefer to draw your line freely, deselect the SnapTo option to turn it off.
To finish the route, simply hit the Enter/Return key and you will see a finishing flag marking the end.
If you would like to extend your line further, you can continue drawing your line by double-clicking on the end-point, and clicking to place another point.
You can add points anywhere on your line by holding the A key and clicking on the line.
Even after you've saved your route, you can always go back in and edit it at any time.
You can delete points on your line by simply selecting them and hitting the Delete/Backspace key.
To delete multiple points, select the first point, hold the Shift key and select the last point. You will see all the points in between these two points are now highlighted and you can hit the Delete/Backspace key to delete all of them.