The most interesting fact about this is that though many instruments had much more than five strings, traditional performers used them very rarely. Sometimes these additional strings even stayed untuned.
When played the instrument was placed either on a table or on laps with the shortest string closer to the player. Playing technique has a lot of regional variations. There are however two different basic principles of playing. Melodies are played with fingers of both hands so that each finger is attached to a single string. Simple tunes were played by picking strings one after another but it was often that several strings were picked at the same time. This allowed polyphony in playing. The other way of playing consisted of damping some strings with fingers of the left hand and strumming strings with the right hand. This allowed to play chords. These two techniques were often combined with each other. Kantele can be played alone as well as used to accompany singing and rarely in ensemble with other instruments.
The basics of playing technique are very well described in: Heikki Laitinen and Hannu Saha. A Guide to Five String Kantele Playing. Kaustinen: Kansanmusiikki-Instituutti, 1988.