I should try some of this exobiology but I'm not sure where to start. Do I just point my DSS at a planet and land in a blue patch?
More or less, yes. After you DSS the planet, you can filter the results (using Q and E on PC). Wherever you see a blue overlay, that's where you might find the bio.
Notice I said might. It can sometimes take some searching. I fly low and slow and keep my eyes peeled (25m high and 25m/s speed is good, so you don't miss the small stuff -- also sometimes things spawn in as you fly, so if you go too fast you'll miss them).
Note that terrain matters, so experience with a particular plant will help you know where to find it next time. The blue overlay is slightly transparent, so the terrain shows through. This can give a hint, once you know what to look for.
For example, I've found that Fungoida usually like higher elevations, so I look for a blue overlay that's on mountainous terrain. You can toggle the overlay off and on by switching to Combat mode and back.
Once you exit Orbital cruise and finish your glide, you'll be normal space and the overlay will disappear. You have to rely on memory after that. If you need to see the overlay again, or change the filters, you have fly back up into Supercruise, turn around, and fire up the DSS again. Then you can find a new spot to try.
I've been finding it to be a lot of fun, and you get to see some cool stuff.