Two players.  (There exist variants for three, four, and more.  This
two-player version is the variant I learnt to play.)

The deck consists of 24 cards, 9-10-J-Q-K-A in each of clubs, diamonds,
hearts, and spades.  It is a trick-taking game with trump.

First dealer is chosen by any means agreeable to the players.

Dealer deals five cards to each player (one at a time alternating,
non-dealer first, or sometimes in other patterns), places the remaining
cards face down between the players, and turns the top card face up.
(The rest of the cards, the ones below the turned-up card, are not used
until the hand is over, at which point they are shuffled back together
for the next deal.)

Non-dealer has the option of telling the dealer to pick it up.  The
exact language varies depending on group, locality, and taste; typical
examples are "pick it up" or "I order it up".  In this case, dealer
picks up that card, then discards one card (possibly, though rarely,
the one just picked up) face down on top of the unused cards.

If non-dealer does not order it up, dealer has the choice of picking it
up anyway, again discarding one card face down, or dealer can pass.

If non-dealer orders it up, or dealer picks it up anyway, the suit of
the turned-up card is trump for that hand.

If neither player takes that option, non-dealer has the option of
naming any of the other three suits as trump, or passing.  If the
non-dealer passes, dealer has the same option (variant: dealer is not
allowed to pass this time).  If dealer passes this second time (if
allowed), the hands are thrown in and a new shuffle and deal occurs
(deal may stay with the same dealer or may switch to the other player,
depending on group, locality, and taste).  If one player names trump in
this phase, no draw-and-discard occurs.

Once trump is decided, two cards shift.  The jack of the trump suit,
called `right bower', is elevated out of its usual ranking, becoming
highest trump.  The other jack of the same colour is `left bower' and
is the second-highest trump; for the duration of the hand, it is
treated in all respects as being part of the trump suit - its original
suit is one card short for the duration of the hand (left bower having
deserted it to become a trump).  Below left bower, the trumps rank in
descending order A, K, Q, 10, 9.

The player who chose what suit would be trump is `maker', the other one
is `defender'.  Maker's goal is to take at least three tricks
(preferably all five); defender's goal is to ensure maker fails to take
at least three tricks (which means defender instead takes at least
three tricks, since there are five tricks to the hand).

Once trump is named, eldest hand leads to the first trick; after that,
taker of each trick leads to the next.  Second player must follow suit
if possible.  This repeats until all five cards from each hand are
played.  If maker takes all five tricks, maker scores two points and
defender none; if maker takes three or four tricks, one point (and
defender none).  If maker fails to take at least three tricks, defender
has euchred maker and defender scores two points and maker none.  In
any case, deal passes to the other player.  (Variant: if maker takes
_no_ tricks, defender scores four points.)