Depending on where you start from, I suggest going east along County Hwy C from Gervais Lake to 1st St. Go south and cross Hwy 36 on the bike trail. From there you will hook up with the Gateway Trail. Take the Gateway to the end, then left on Norell, right on Square Lake Trail, left on Norell, right on 170th, left on Olinda Trail. Everything to this point is on the Twin Cities Bike Map.
Olinda Trail (Cty Rd 3) goes north through Scandia to Panola Dr (Cty Rd 86). Turn right and go east to Hwy 95. Left on 95 to Osceola Rd (Hwy 243). Turn right. This will take you across the St. Croix River into Wisconsin. You have a number of road options from here, but your goal is to reach the Gandy Dancer Trail in St. Croix Falls. The trail is crushed limestone. It can be ridden with road tires, but if you prefer pavement, most roads in this area are paved, even the smaller town roads.

My knowledge of this area is weak, but I would suggest riding north to Centuria, then taking back roads east to Cumberland.

Thanks