Go to Parmer & I-35. The Parmer exit is in north Austin, several exits north of 183.
Go west (left) on Parmer.
Very soon you will reach Lamar. Turn south (left) on Lamar. (Note that Lamar is also known as Loop Something up here, and may not be labeled as Lamar. You'll know it is Lamar because it is the first road to the west of I-35 along Parmer)
After about half a mile (you will pass one traffic light at Yager Lane on the way), you will make a right turn into a park. There is a sign just before the turn that says Bob Havins Fields on it, but the sign is not large and the intersection is not at a traffic light. If you hit Braker, you've gone too far.
Once you've turned onto this road, the fields will be on your right after a little while. They are visible from the road.
Unlike the Krieg fields, there are only 5 fields at Havins. If you enter the park from Lamar, fields 4 and 5 will be the first softball fields on your right. If you drive a bit further up the road, you will come to fields 1-3, also on your right. The field numbers are on the backstops of the fields.
It is easy to miss the turn off Lamar, so please leave yourself plenty of time to get to the field.
If you want to map this on Google maps, the address of the fields is:
12138 North Lamar Boulevard
Austin, TX