Nice, Jay. Lots of good advice here already, so I'll keep this brief. If you're going to spend cash on sound equipment, you could do worse than getting a basic Rode shotgun for your main camera, i.e. the one you're pointing at the person / animal you're featuring, so it's a good bet you'll get their sound at least. And if you can afford it, pick up a Zoom H6. They're good quality, easy to use, and have several removable mic heads, so you can choose one for more ambient stereo sound, one for targeted shotgun style, and a versatile XY that you can tweak really easily for more targeted ambient sound, like kettles and sheep. And a decent pair of headphones, of course, so you can hear what you're supposed to be recording, and can tweak appropriately. The H6 would also enable you to plug in on its own track a cheap wired lav mic, which is a lot cheaper than the wireless types, and would work together with your shotgun, each being the backup for the other.
Put your H6 ambient-ish one near the action. Start everything recording. Do your clap (on camera, nice and loudly and cleanly) and announce what the heck you're filming, so you can quickly identify your sound files later. Like Jay said, it's a quick easy way to sync everything up. Recognise that whatever is in front of your mic will include whatever's beyond your immediate subjects, so a simple shift of angle for your ambient mic may make the difference between a frustrating edit and a 'good enough' edit.
If you just keep recording, instead of lots of start and stops, then you only need to deal with the initial sync and you're good to go. The downside is that you'd be increasing file sizes with stuff you don't necessarily want.
If you find you don't really want to keep doing it after a year, I'm pretty sure you'll be able to sell the H6 and Rode for not much less than you paid for it, since they're known to be reliable, good quality gear, especially if you pick them up used to begin with.
And finally, learn to listen consciously.