Like anything there are many methods - I quickly filmed two of them.
Train your dog to target their front paws (there are loads of videos on this) then have them target your arm with either paw then both paws
With your arm out in position lure the dog up - they will then rest their paws on your arm
Simply feed your dog his treats between his front legs, the more you feed in this position the more the dog will enjoy being in this position and go into it naturally in anticipation of the reward
repeat until you presenting your arms becomes a cue in itself to do the trick
This is a nice cute trick that kids love to see - it is also good for building some muscles being a step towards doing a begging trick