meditation is very necessary in ninjutsu or almost all martial arts.Meditation is daily routine for most eastern cultures,the modern west is still not very open to meditation but as far as martial arts go,it is very very necessary.As once said before,techniques are mere manifestations of the mind,we control our arms with our mind,and our arms control the technique,stronger mind=stronger technique.
to strengthen mind...meditate...simple philosophy that has served others well...
As far as "how" goes this is really varied,from person to person,school to school,art to art,religion to religion...there are so many different ways.some people just sit there,lie there or stand and control their breathing,others get complex and do "moving meditation" or hand positions and so forth...it depends on alot of factors and the individual....
much respect
-andrew