Teaching a tuck sit is easily achieved through luring and if practiced a few time each day you could have it on cue in a few days, then be working on generalization! While you can add a cue word right at the beginning of training a sit, I prefer to wait until the dog can perform the behaviour for an empty hand lure to ensure it isn’t just the food the dog is responding to.

On cue dog should sit with the rear end on the ground and front feet stationary.