How to read the paper and the normal waveform - the blueprint every later rhythm is measured against. 1 small box = 0.04 s (time), one large box = 0.20 s.
One normal beat with its parts labeled: the P wave (atrial depolarization), the narrow QRS (ventricular depolarization), and the T wave (ventricular repolarization). PR 0.12-0.20 s, QRS 0.06-0.12 s, QT measured from QRS start to T end.
Reading ECG paper: each small box = 0.04 s wide; five small boxes (one large box) = 0.20 s. Count R waves in a 6-second strip x 10 for rate. Here a regular ~75.