Noun Verbs/Verbal Nouns
Some nouns can be used as verbs simply by following them with the basic verb する, which means to do. Generally, they can be used with or without the を particle in between the noun and する; but if a direct object is involved, there cannot be two を particles in the same clause so one must be omitted. The easiest way to do this is to drop the を between the noun and the する but in many cases, the を corresponding to the direct object can be dropped instead.
CORRECT: Tennis wo renshuu shimasu. (I practice tennis.)
CORRECT: Tennis no renshuu wo shimasu. (I practice tennis.)
WRONG: Tennis wo renshuu wo shimasu. (I do the practice tennis.)