what are the principles in obtaining the radial pulse rate

What Principles? You just take your pulse on the inside of your wrist with your fingers (no thumb).

Radial pulse is felt by light compression of the radial artery over the lower end of radius, in front of the wrist. It should be counted for at least 30 seconds. The wrist should be relaxed to get a good pulse.

Radial pulse is assessed further by a method known as trisection or Osler is manoeuver (compression using three fingers). Compression by the first & last fingers occludes flow from radial artery & retrograde flow from ulnar artery through palmar arch. This helps us to assess the vessel wall thickness, which is done by the middle finger.