I have built a low cost EC sensor using the Atlas Scientific probe. The EZO board for the pH sensor has the same interface so if you look on my recent blog I give all the details you should need. I assume you want to deploy this with battery power and solar recharge so the Mayfly will sleep most of the time. Setup code is included in my blog and you will want this initially to set up the Atlas EZO interface board which provides serial comm to the Mayfly.
Hope this helps