I just did this over the last week or so, and I'm wishing I'd either sprung for the ofeeshal crimpers, or else soldered the dang things. Using regular pliers, I was too wimpy to get good connections and wound up beating on them with a hammer, which mostly worked but I still have two shelves that are not heating up.

Bean farm sends a little flier that made things clear for me.

Things I learned the hard way:
Wire in paralell, not serial. Serial made sense in my head when I planned it out for the first time. Parellel is less work, less materials, and much easier.

Do as much as you can outside the rack. For purely cosmetic reasons, I kept my sidewalls solid, and ran the wiring down inside the rack through notches cut in each shelf. Big huge mistake. I spent far too much time bent over reaching to the back of the shelves fumbling blindly to make connections. Never again. Next one I do, I'm going to drill holes in the side walls and run all the wires outside, with connector clips so I can assemble everything at a work station and then put it in place.

Also...those little bitty plastic insulating covers are much bigger than they look; allow for 1/2" of extra shelf length for each one. If they come apart again once snapped together, I've yet to find the trick. When some of my connections turned out dodgy, I wound up hammering at them with pliers (blindly, at the back of the shelf, on my knees at an awkward angle) until they smashed enough that I could pry them apart and get at the connection. They are sturdier than they look as well.

Had I run the wiring externally, I could have just unplugged that section of flexwatt, moved it to a work area, and corrected the problem...as it was, it was a huge PITA to fix, and I still haven't gone back to tinker with the bottom two shelves.

And with the bulkiness of the plastic insulators, I'm wishing I'd soldered the connections instead. I saw a FAQ on it, but was too intimidated by inexperience to feel comfortable trying it. In retrospect, it would have made a much tider and more secure connection.