It's easy to install, just press the release buttons on the underside of the Apple Watch, slide the original straps out and insert the Kardia Band ones (the sensor band goes into the one at the bottom of the Watch).
The watch rebooted after the sensor strap was inserted, but it came back fine.
You need to have the Kardia app installed and register an account, then make sure it's installed on the Apple Watch.
Running the app on the phone doesn't do much (it's designed to work with other AliveCor products). Opening the app on the watch then gives you the option of recording an ECG. It's best to have your hand resting somewhere and not moving much (there are options in the app to select which region you're in and whether you're in a 50Hz or 60Hz mains area).
The sensor strap has two sensors, one underneath the strap and one on top. Say the watch is on your left hand, lay that flat somewhere and place the right hand on to it and your finger on the top sensor. Hit record (well you'd probably do that before, you're given some time before a reading is taken) and a countdown timer starts, stay as still as possible until it finishes. After the reading is taken is does some calculations and you can scroll through your ECG and the watch app tells you if it's normal or not (couldn't test the not normal reading) and you can save the results.
It's possible to allow another user to use the app, but the watch needs to be placed on them.
The band costs £99.00 direct from the AliveCor site in both 388mm and 42mm versions. It's a nice quantified self device to have, but expensive and unless you have a heart problem, don't know if really worth it and have to wait to find out how it affects battery life of the watch.