Going straight from the originals, ok? Two-centing, as usual 
If you had looked before crossing the street, you would not have been hit by a car.
Si hubieras mirado antes de cruzar la calle, 
no habrías golpeado por un coche.
Make it active, sounds better
el coche no te habria dado un golpe / no te habria pillado
If they had not thrown out all the donuts, there would have been one dozen left over.
Si ellas no hubieran echado a la bastura todas las dónuts,
You don't need the subject, I would just drop ellas
in spain we usually say tirar a la basura, not echar, though it may be fine somewhere else, I dunno.
habría sobrado una docena.
If I had seen her leave the house wearing that outfit, she would not have made it past the front door.
Si yo hubiera visto que ella hube salido de la casa mientras estaba llevando eso conjunto, ella lo no habría hecho pasado la puerta principal.
Whole rephrase from scratch, better. Too much word-to-word translation
Si hubiera visto que ella salia de casa llevando eso/vestida así, no la habria dejado poner el pie en la calle.
Notice the shift from the front door off to the street, interesting, eh?
If he had been angry enough, he would have reacted differently.
Si él se hubiera enojado bastante, habría reaccionado de otra manera.
Enojarse is an American form. If you keep it, the sentence is fine, I guess.
In peninsular spanish the expression tends to be emphasized, and the first clause would probably be turned into negative, since the connotation is negative...
Si el no se hubiera/ese cabreado/enfadado lo bastante
Elaborating: just by dropping a como at the beginning, you turn the sentence into an exclamation:
¡Como si el no se hubiera cabreado bastante! As if he ...!
If they had paid attention, they would not have gotten themselves lost.
Si ellos hubieran prestado atención, no se habrían perdido.
prestar atención/poner atención
Possibility: turn the polarity
...., habrian llegado a casa sin problemas.
If we had understood how to put this thing together, we would have been able to do it.
Si hubiéramos comprendido de qué manera esta cosa está ensemblado, habríamos podido hacerlo.
I'd put the second clause upfront
Habriamos sido capaces de hacerlo si hubieramos comprendido/entendido las instrucciones
Notice the simplification of saying 'las instrucciones', more effective communicatively
If there had been a snowstorm yesterday, we would have made a snowman today.
Si hubiera habido / caido una nevada ayer,
Verbalise nevada
Si hubiera nevado ayer
habríamos hecho un muñeco de nieve hoy. (hoy sounds obvious)
If you had taken the english class this semester, you would have had the same teacher as my son.
Si hubieras tomado la clase de inglés este semestre, habrías tenido la misma profesora como mi hijo.
Si hubieras elegido/escogido ingles este trimestre / como trimestral, habrias tenido la misma profesora que mi hijo.
If I had added sugar instead of salt, the cake would not have been ruined.
Si yo hubiera añadido azúcar en lugar de sal, el pastel no habría arruinado.
Whole rephrase from scratch, first part is acceptable but wordy.
Si no me hubiera confundido con el azucar (y la sal), no me habria cargado el pastel.