Here's a helpful YouTube video that RideWithGPS put out for you to preview, named “Route Planning for Club Members Recording”. The link starts the video at 5:06 (306 seconds), and the pertinent section we recommend ends at 21:46.
https://youtu.be/rqpd0XwT8oU?t=306 And here is an accompanying PDF document: RideWithGPS route planning notes and video.