From your direction, M5 and then A38 which means keep to the right as you leave the Exeter area and follow the signs for Plymouth.
Go past Buckfastleigh and get off at the Marley Head exit, signposted for Totnes. Cross back over the A38 and head into the Dartmoor national park.
About a mile or so and you come to a staggered crossroads, go straight over. Another mile or so and the road bends left very sharply, and there is a minor (even smaller road!) exit in front of you. Take that exit, don't go round the bend.
The road goes down and down to the bottom of a valley; cross the bridge at the bottom and the ground will be found on your left.
By that time the jolly marksmen of Dartmoor will have put out some signs, only in English though. Not sure how you write FT in Welsh.