There are many "recipes" floating around on the net for you to try. In addition, there are calculators that tell you the %s of what + what = final product.
For example:
http://www.ecigvape.com/simple_calc.php
Base nic liquid strength in their example is 36
You want to cut it to 18.
Add 4 ml of pg/vg to 5 ml of the unflavored nic juice.
Add 1 ml of flavor.
that would make 10ml of 18 mg strength e-liquid.