Make sure you have turned off Auto Pause. This can be done in the Watch or in the My Watch App on the phone in the Workouts section
Learn how to lock the screen against accidental tapping by hand or loose clothing consult this article
Use the Workout app on your Apple Watch - Apple Support
Make sure the Watch is snug fit - you should get pinkie between strap and wrist but not easily your thumb.
Skin pigmentation and Tattoos will affect the light transmission required by the wrist detection and HRM on the back of the watch
Wearing on the other wrist or further up the arm may help but there is no other workaround for tattoos which affect all makes of sports watch