Dilute the antiserum with a buffer that is used for the conjugate or a suitable buffer for the other materials in the assay.

Dilution must be done with buffered saline.

I agree with Sourabh and Jack. However, I would go one step ahead by saying that do use buffer (not buffered saline) but in pretty low concentration.

You can’t dilute your antiserum and conjugate with water. You should dilute your antisera with coating buffer and your conjugate with conjugate buffer. You can get easily these buffers preparation if it is not in your hand. However, I suggest that you can decrease your reagents (IgG and conjugate) quantity half even quarter instead of dilution. You can use your antisera and conjugate 100µl,75µl or even 50µl rather than 200µl. It is better way than dilution.