I don't really understand what "the seconds" means. Many EKG strips you review will not be six seconds, exactly, in length, so multiplying the QRSs in one x10 may or may not give you an accurate rate. Also, six seconds of any irregular rate won't tell you much.
Knowing what the EKG measures and how will help you understand what you're looking at. EKG recording started in the golden age of physiology research in the late 1800s-early 1900s when Einthoven figured that the cardiac muscle, like skeletal muscle, had something to do with electricity. He stuck electrodes under the skin of willing volunteers and looked at the spikes that resulted. They looked different depending on where he stuck the needles, and if he pulled the paper underneath the moving recorder the spikes could be timed and counted.
Now, you can't go sticking needles in people like this, so his next stop was to put the people in pickle barrels full of brine (an ELECTROlyte solution, means..."conducts electricity" Aha!) and stuck the electrodes around the barrel. This way he could standardize his recordings so he could compare them to one another.
Well, you can't go sticking sick people in pickle barrels, so eventually they figured out how to use the skin electrodes we have now. After a lot of physiology researchers got into the act, they decided to standardize their methods so they could compare them all. That is why you now have EKG paper with the little squares on them, arranged in big boxes of 25 small ones. The paper goes under the recorder at a fixed rate such that each small box = 0.04 seconds, so each big box = .20 seconds. So there are 300 big boxes in one minute. SO if you have a beat that happens in every big box, that's a rate of 300/minute (eek). Every other box = 300/2, 150/minute. Every third box, rate is 300/3, 100/minute. Every 4th box, 300/4, 75/minute. See the pattern? Of course this works for regular rates or regularly irregular rates only. Irregularly irregular rates, you're better off counting for a minute.
The up-and-down axis is voltage, as in, how much electricity is being measured. Great big fat hypertrophic ventricles that generate a lot of electricity in that muscle mass make great big spikes (seen in chronic hi BP, for one). Sick, crappy, thin ventricles that have very little oomph make smaller ones. This too is part of the standardization of EKG recording. There will be a square wave somewhere in there that will show the calibration.
About those electrode placements-- what's that all about? Well, you will remember that electrical current passes through the heart along a regular pathway (normally), from SA node thru the atria to the AV node and through the ventricles, left and right bundles. This makes a nice coordinated contraction, emptying atria into ventricles and then wringing the ventricles out like a washcloth to squeeze the blood out to the aorta and pulmonary artery. We sometimes need to know where those pathways are disrupted, because that could be an indication of pathology in the related area.
Imagine you are at the baseball park in Philadelphia, and the best pitcher in baseball, Steven Strasburgh, is throwing a sinkerball-- it goes straight from the mound to the catcher, but drops down as it approaches. Imagine you are at first base looking at the path of this ball-- it appears to go from your right to your left, dropping at the end. If you had to draw that, it would be a dropping line from R>L. If you are standing at third base, it appears to go from your left to your right, dropping, so your line would be L>R, dropping. If you are standing on second base (not playing second), pretty much all you'll see is a small dot that drops but it's hard to see where, because you're not in a good position to see the whole path of the ball if the ball is moving away from you; that line would be very short, and drop a little bit. And if you're in the rafters looking down at the field between the mound and the plate, you'll see a ball going straight from the mound to the catcher but you won't see the drop at all, just the straight path, so that line might go top-to-bottom of the page, straight.
This is what the different electrodes do. The give you pictures of the electrical motion (the ball) but you see different waves depending on where you look. As it turns out, there are better places to see the ventricles, or the atria, or subsets of all of these. Part of learning how to read a 12-lead is knowing what leads, or groups of leads, look at what part of the heart best. I will let you look those up...but a hint: precordial leads, the ones on the front of the chest, look at the anterior heart; one lead group includes AVF (for "foot") and that looks at the inferior part (closest to the feet)... check it out. That's how they know it's an anterior infarct or an inferior one, or a septal or lateral, by what leads show the characteristic changes of ischemia and infarct best.