There are resources out there for doing Nature Study with your homeschool student. We've tried some of them, and they were fun enough, useful enough. But they didn't work out for us over the long-term, because the book/journal/websites invariably don't reflect what is actually happening in our backyards, on the trails we hike, or out our windows.
(We encountered a whole bunch of kinglets on a recent hike. They almost seemed to want to keep pace with us as we walked.)
So now I don't really plan nature study in advance anymore. The only "plan" we really need comes down to:
1. Set up observation stations meant to attract something. Mostly, this means bird-feeders. It can also mean putting out bat boxes, etc.
2. Get the kids set-up with: guidebooks, cameras, binoculars, magnifying glasses, a flower press.
3. Regularly spend time outdoors. In the yard. In the park. Hiking. On the water.
4. When you are out and about, be observant.
That's about it, and it's working for us. :)
(Outside my kitchen window I have five birdfeeders.)
That said, if you want to do something less casual than we do, I'm incredibly impressed with The Handbook of Nature Study. Barb McCoy has some great ideas and resources, and you should check it out if you are a homeschooling parent. Let's get all the kids outside more, and in touch with nature!