Source: Acción Gramática
Take the first person, and modify it for the whole conjugation. When learning a new conjugation, I prefer to use first, second, and third (I, You, and He/She/It respectively) first.
Present
Verb stems
Compro Bebo Subo
Pienso, Vuelvo, Juego
Prefiero, Duermo
Pido
Imperfect (Past)
Preterite Stem + Ending