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Hold a treat in front of their nose then move it away horizontally so they can still see it and would walk towards it. At the same time gently put your hand under their tummy/ waist area so they stand. Reward with the treat, eventually introducing the word and making them wait longer for the treat.
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^^^ This. I found it easier from a sit and no need to encourage with touching if you use high value treats (not kibble). Moving the treat away horizontally (not too fast) means pooch will (should!) automatically stand to follow the treat. You'll know how far to move the treat after a few attempts. Click and reward as soon as fully standing, not just with bum off ground. Make sure pup is stationary and not dicking about before clicking and releasing the reward. Once the penny drops, wait a little longer before rewarding. Rinse and repeat, but only a few minutes at a time initially.
However, I think the OP is referring to standing still for showing which is a different kettle of fish altogether. This is something that will take months of patient practice (pups by definition don't/won't stand still for longer than a second), but practicing a normal stand command is the first step.