First, you put in your starting point and destination so Spotify can calculate your drive time with the help of Google Maps. Then you're led through a series of questions that gather info like who you're traveling with, your favorite genre for a road trip and what type of car you drive. The questionaire also asks for your "drive vibe" (mellow, sing-a-long, pedal to the metal, etc.) and your "ultimate road trip song." The list there is limited to six options, and nearly all of them directly reference driving, so you have to go with one that's closest to your preference and keep on truckin'.
When you're done, Spotify will compile a playlist that lasts the length of your journey, and save it to the playlists section of the app. Soundtrack your Ride is available on the web via both desktop and mobile devices. Initially, some of the options were cropped off on the mobile site, so you couldn't see all of the possible responses. The company updated the mobile version and now everything can be read easily. This means you don't have to finish the quiz before you depart, or remember to plan ahead.
Update 11/18/19 3:44PM ET: Spotify updated the version of Soundtrack your Ride that's available on the web from a mobile device. This post has been updated to reflect this info.