Poor skin prep leads to unwanted artifact and not placing the electrodes where they should be can change the morphology (shape) of the waveforms the doctor is going to interpret.
The skin prep is as important as any other part of this procedure. A poor skin prep will yield poor results (artifact laden). Apply a moderate amount of pressure when prepping the skin.
I use a circular motion when prepping the skin. The intent of a good skin prep is to remove dirt, dead skin cells, oils, skin moisturizer, fake tan, body powder, sweat etc. All of these sources can lead to poor contact with the sensors and artifact.
Placement of the electrodes is very important when performing a 12-Lead EKG. When placing limb sensors, I prefer to stay away from large muscles due to the potential for movement and increased somatic tremor.