I'm a moderate DIYer and I had a great time doing this. Here are a couple things I learned. Hopefully this will help someone:
1. When I first started setting up zones, I tested my first zone (front door) saw it work and felt great. I proceeded to connect the next 5 on that board and none of the zones after zone 1 were working. I started troubleshooting like crazy. I plugged in other wires to the working terminals and they worked. Plugged in the front door to terminal 6 and that worked. I tried my add-on board and all terminals worked. I thought I had found a bizarre problem where only one zone at a time would work on my main board. Turns out I was just impatient. My Smartthings app was taking a while to get up to speed. I was trying hardware solutions so quickly I didn't realize it was app related. So if you connect zones and they seem funky, give it a minute. Force quit your Smartthings app. Reboot the board, take a beat. It could be a network discovery issue rather than a hardware issue.
2. While troubleshooting that board, something did go wrong. Suddenly the main board got very hot and the blue light on the wifi card went out. Other cards worked with that same board, so either I fried the wifi card or it was faulty. I emailed Nate and he said it has happened to other wifi boards. So he sent me a new one quickly, which I really appreciated.
3. Think about where you want to position these things and keep an eye on your cables as you go. It's a pain to try to untangle cables after you've connected them to the boards. Also, on a 4-board system, it's going to be hard to organize the cables and get the boards into position so they can be powered by the DC splitter. I found it not worth the stress. I went to Amazon and got three pigtails. Nate sells them here too. I think it's a much better way to go than using the splitter. Power one board from the battery, then power the next one from the DC output of that board. And so on. It gives you way more positioning leeway.
4. Don't make my phone mistake. My old system had a landline coming up into the box. It was wired to a standard phone box (rj11, is it?). I figured let's just get rid of that clutter. I cut the box off the wire and bundled the wire up out of the way. Our landlines in the house went out. No dial tone. Turns out, you can't just cut a phone jack off its wires and leave them there. I ended up re-connecting the phone jack. I think actually I could just connect the wires to each other, red to black and blue to yellow, but I didn't want to jinx anything. I just reconnected the box.
5. I labeled my boards with a sharpie, so whoever has to decode this in 10 years will have an easier time.
That's all the advice I can think of for now. Have fun. It's a great project for a light DIYer like me.