Sound the note
In order to sound the note, you can use an electric tuner or piano to tune the second main string to middle C. While you are holding the sitar in a playing position, the second string from the top side would be the second main string. For the key in which the sitar is to be played, this string is tuned to the tonic note.
You should now pull the string up a little and stretch it before retuning. This process needs to be repeated until the string is tuned according to your choice.
Tune 4th, 6th and the 7th string
While you are holding the sitar in the playing position, you need to count down from the top to identify the 4th, 6th and the 7th string. In this case middle C, you can now tune these strings to the tonic. Just like you did with the second string, stretch and then re-tune each one of these strings one by one.
Tune the first string
You should now go to the first main string and set it to the F above middle C using the same method in order to properly tune it to the key of C.
Tune the sympathetic stings
Having tuned the main strings, you should now come to the sympathetic strings and start to tune them one by one, starting from the shortest one. To the notes of a C major scale, these strings need to be tuned in order of their length.