You can download trip waypoints by clicking on the "gpx" link on the Trip page. (same thing for roads and trails). When you click the link, it will ask you where you want to store the .gpx file on your computer.
Once the .gpx file is on your computer, then you need to copy it to your GPS. Almost every GPS has a function to import GPX files.
Direct "Open" of gpx on phones:
If you are using a phone based GPS app such as Backcountry Navigator, you can do the whole operation right on your phone, without involving your computer. Just browse Bivouac on your phone, find the trip you want, then click the link. Your phone will download your file, and then display a link called "Open". When you hit that link, it will display the programs with which you can "open" a gpx file.
The Bivouac waypoints become a track log on your GPS.
Download road from Bivouac to your GPS