Take Hwy 121 all the way to Hwy 289 and head North.  289 ends or joins with Preston Road.  Continue on Preston Road until you reach Eldorado Pkwy.  Turn left on Eldoradoa and the fields will be on the left.

Alternative instructions: Take I-75 N to SH 121. Go west on SH 121 to Preston Rd.. Turn North (right) on Preston Rd (SH 289) to Eldorado Pkwy. (Eldorado is approx 5.6 miles from the intersection of Preston and 121...Note that Eldorado is "where the center turn lane ends.") Turn left (west) onto Eldorado Pkwy, go through stop sign at Rogers Road. Turn left into Warren Sports Complex.

Here's an area map showing Frisco. 

Here's a blowup view of Frisco.  The fields are labelled Ward Sports Complex on the map but I believe that may be incorrect.  Our fields are called Warren Sports Complex.

Here are maps of the fields.  Click on the image below to view a full size version of the field map.