You simply need to point your mouse cursor onto the edge of the already existing building piece in order to snap additional build items to it. I dont know if that is possible with the "field fences" (if you mean those) so good luck with them (my field fences always look a little bit run-down but i dont care). Most building pieces cause little to no snapping problems if you know how to do it (and if you dont want to build complex, intersected building parts).