How to make a lifesteal enchant?

create the enchantment called life steal
create a procedure with the trigger entity attacked
  if the item in main-hand of source entity has the life steal enchantment
    set health of source entity to source entity health + amount 

you can also make this multiplied by your life steal level if you want, or have it be (amount/2) x enchant level 

you can get amount by double clicking the dependency on the right side when inside the procedure