The basic post describes yoko ukemi in general, but here you will find way more details.
Details
– Body (before leaving the mat): Keep your body straight (to not land on your hip first)
– Body (before hitting the mat): Bent your body at the hips (to reduce the impact on the legs)
– Head: Tuck in the chin before hitting the mat (prevents the head from hitting the mat)
– Slapping arm : Slap the mat quite hard with an angle of 45% (reduces the impact on the body)
– Other arm: Try to keep a grip on your opponent, e.g. lapel (for better control and less impact)
– Pelvis: Try to land on the same side butt cheek and not on the side (hip) or the back (spine)
– Upper leg: Keep a slightly bent leg and try to keep the leg up in the air (protects the knee)
– Lower leg: Land on the outside of the slightly bent leg and scoop the foot upward (protects the ankle)
Caution: Don’t strike/slap the mat with the legs as you do with the slaping arm
Example (Harai-Goshi)