The abilities spliced onto the spell happen last, after all of that spell's other effects.

If, during a player's declare attackers step, a creature that player controls that's been goaded is tapped, is affected by a spell or ability that says it can't attack, or hasn't been under that player's control continuously since the turn began (and doesn't have haste), then it doesn't attack. If there's a cost associated with having a creature attack a player, its controller isn't forced to pay that cost, so it doesn't have to attack that player.

Being goaded isn't an ability the creature has. Once it's been goaded, it must attack as detailed above even if it loses all abilities.

If all of the spell's targets are illegal when the spell tries to resolve, it won't resolve and none of its effects will happen, including those from cards spliced onto it.

If a creature you control has been goaded by multiple opponents, it must attack one of your opponents that hasn't goaded it, as that fulfills the maximum number of goad requirements. If a creature you control has been goaded by each of your opponents, the creature must attack an opponent (rather than a planeswalker or battle), but you choose which opponent it attacks.

If a spell is copied, choices made while casting it are copied, so the copy will have the same abilities spliced onto it as the original.

Attacking with a goaded creature doesn't cause it to stop being goaded. If there is an additional combat phase that turn, or if another player gains control of it before it stops being goaded, it must attack again if able.

Any cards you splice onto a spell remain in your hand, regardless of what happens to the spell after you cast it.

If the creature doesn't meet any of the above exceptions and can attack, it must attack a player other than the controller of the spell or ability that goaded it if able. If the creature can't attack any of those players but could otherwise attack, it must attack a planeswalker an opponent controls, a battle an opponent protects, or a player who goaded it.

You reveal all cards you intend to splice at the same time. Each individual card can be spliced only once onto any spell, although multiple cards with the same name may be spliced onto one spell.