When using an Elemental Jutsu, defined as a Jutsu that has one or more affinities in the requirement field, you can reduce the chakra cost based on the Elemental Affinity you have. This is determined by taking you Elemental Affinity Level and dividing it by 5. This number is the amount you can subtract from the jutsu depending on rank.

S-Rank No more than 1/4 of the Chakra Cost
A-Rank 15 Point Reduction
B-Rank 10 Point Reduction
C-Rank 5 Point Reduction
D-Rank No Reduction

So below is an example.

Name: Kirigakure no Jutsu (Ninja Art: Hidden Mist)
Description: A ninjutsu technique that covers the area surrounding the user completely in mist. Once used, there's practically no visibility within the area of this jutsu for both the user and their enemy. This is a perfect situation for someone who trains in the art of silent killing.
Type: C-rank, Supplementary
Activation Cost: 15 Chakra
Hand Seal: Modified Hitsuji (Ram)
Requirement: Wat(15); Random roll Jutsu Power. Result must be more than 5 to successfully execute.
Effect: Opponents attack, defense and speed is reduced by 1/3 (round to the nearest whole number), for 3 turns.

This is a Water Based Jutsu because in the Requirements field it has Water:15. So I can take my water affinity(lets say 30) and divide that by 5 to get 6. Since this is a C-Rank, I can only reduce the chakra cost by 5 so I lose the 1 extra point and the new Chakra cost is 10 Chakra.

If you are using an advanced element (a combination of two basic elements) you take the lowest affinity to use in it, unless this is granted by a bloodline like the Kurowashi Clan and the Paper Based jutsu. If it is a bloodline then you get the average of the two, so add them together then divide by 2, and use that to determine the chakra reduction.