Many have hit on the most important part, drive. If you can figure out what that dog will do anything for...a tennis ball, a treat, or a simply belly scratch...well, then you can likely train them to darn near do anything for that reward.
If the dog just doesn't get excited about anything, then they are a poor candidate for trailing. They'll lose interest fast, because their reward just doesn't mean much to them.
To train, just take bottle or leak-proof bag of some kind, next time anyone you know is about to gut a deer. Cut it open slowly and hold the container under the incision and try your best to get all the blood you can. Then get home and freeze it in some small vials.
Then you can just pull out a little bit at a time to thaw out and train. Start by dripping some on the open ground...point to it and give some command, think "find it" or "deer" ore whatever. When they sniff it the very first time, and hopefully wag their tail some, give a lot of praise and immediately give them their most prized reward.
Slowly work up to them going from one drop to the next, to maybe a 30-50 feet, to a piece of deer hide or antlers...when they reach then end, they get their reward.
Before long, that dog will be on the trail before you can tell them, all to get some silly reward.
(It's the same process for shed hunting. My lab is crazy for antlers, never found a real shed but did find some dummy plants 2 days after I lost them in the deep snow last year....the bird work is way behind for her, she's just way to high strung!)