You should be familiar with the rules of Euchre before playing this game. Like standard euchre, the top card of the remaining three is flipped up and placed on the pile in front of the dealer. Play is very much like standard euchre.
Trump is determined the same way.
The person who calls trump is allowed to call their partner, if they wish. They call their partner by naming a non-trump card. If they are ordering the dealer to pick up the trump card, they will reveal their called card after the dealer has discarded, to prevent the dealer from discarding the called card intentionally. The player that has the called card is their partner. However, they cannot reveal that information until they play the called card, so it will not be immediately apparent who the caller's partner is.