For back up I sat in front of Thor and waited. The moment his back foot/feet moved I would mark yes (or click) and toss the treat between his front legs so he would have to move back to get it. The moment he moved back to get the treat I'd mark yes and toss another treat between his legs (placement of reward is really important in some tricks). If he turned around or ducked his head down to get the treat I wouldn't do anything. He caught on fairly quick. Once he started offering to take a few steps back on his own I added the cue word Beep Beep and then gradually increased the criteria.