Are you close enough to an REI dealer to let their shop handle the chain installation? Might be easier and cheaper in the long run. Especially if you don't have the original chain to use as a guide for the replacement.

When I replaced the chain on my long wheelbase comfort hybrid I just counted the links on the original. Two or three times to be sure. It was so worn and elongated I couldn't just lay the old and new chains side by side for comparison.

I bought the replacement chain at REI and told the mechanic I needed a 122 link chain. KMC chains come in boxes of 116 links. He just gave me a couple of lengths of new 6-link chains cut from new chains installed on other bikes. I spliced 'em, put on the correct Missing Link, no problems with fit.

It skips a bit in gears 5-7, but that's because the original Shimano MegaRange freewheel is worn out too. I'll replace it soon. Didn't realize how much I'd ridden this bike, especially on gravel and gritty rural chip seal, so the moving bits wear more quickly.