I'll give you the highway numbers and you can follow on a map. I'm a long haul driver so I've been there many times. There's 2 ways...
1- From Calgary and north cross the border at Portal, ND take 52 and 281 to Jamestown and hit I-94 east then I-90, I-39, I-74, I-57 then I-24 into Nashville
2- From south of Calgary cross the border at Coutts/Sweetgrass...I-15, 87, 191, 12, 3(Billings can be kind of confusing to get thru but follow signs to get to I-90), I-90, 212, 85, I-90, I-29, I-70, I-64 and I-24.