To record your ECG place your thumb on the metal blob and your fore-finger onto the other side of your wrist. Run the Kardio app, sit back and RELAX. It's VERY sensitive to any kind of movement. After 30 seconds it will beep and then press save and it will try and filter out your ECG. If you move or there's lots of electric interference it's unlikely you'll get a good reading. Assuming you're not moving around much and the software processes the data, you'll then get a normal (or not) reading and you can scroll through the waveform, like it's been printed on paper and you move along along it.
AliveCor also make a Bluetooth version where you place your fingers on which is actually considerably cheaper and I'd guess gives better readings, but then you have to remember to take it with you or buy their special iPhone cases which wil hold the device.
The Kardia band does work and it's not reported any abnormal heart readings, which is probably a good thing. It's nice that you can take a reading whenever you like (as long as your wearing your Apple Watch), but it's very sensitive to environmental conditions.
The Kardia Band costs £199 in the UK directly from the AliveCor site (it's not available in the US as it's not FDA approved). The watch strap and metal insert have been improved since earliy version when the metal bit tended to fall out.
The Kardia Mobile Bluetooth sensor is available for £99 from AliveCor and is also available in the US.
