Eric from Canada - 4/29/2021 18:42

There is an app for android phones/tablets called Farm Navigator, it’s free with a few ads or you can buy it for a few bucks. It uses your device’s gps to let you set A-B lines and give you a lightbar. Obviously your phone’s gps isn’t all that precise, but you can get a bluetooth gps receiver like the garmin glo that will be much better. For a couple hundred dollars you can get a cheap android tablet, bluetooth gps receiver, tablet holder and a charger.
Try the app out for free if you have an android device, or try to borrow one from a friend. You can get an idea of how it works just walking around a parking lot, large yard, etc.

If you are really ambitious you can build your own autosteer system for around $1000, look up AgOpenGPS. They have a community of DIY people who developed open source software and have shared hardware lists for how they built their systems.