If you cast a spell with demonstrate and both you and an opponent copy it, the opponent's copy will resolve first, then your copy will resolve, and finally the original spell will resolve.

If you cast the spell and choose not to copy it, no opponent will get to copy it either.

If the spell had an X value chosen for it as it was cast, the copies will have the same value of X.

If any additional or alternative costs were paid to cast the spell with demonstrate, the copies will see those choices as well. For example, if the spell has a kicker ability and was kicked, the copies will also be kicked

Resolving copies of permanent spells become tokens as they enter the battlefield. These tokens are not considered "created" when this happens.

You choose whether to make a copy as the demonstrate ability resolves. This happens before the original spell resolves. Your copy goes on the stack above the original spell.

If you copy a spell with demonstrate, you then immediately choose an opponent. If they copy the spell, it goes on top of the stack.