autoprepararse

A straightforward reflexive verb for preparing oneself for something — an exam, a competition, a role — usually implying doing so independently, without a teacher or coach.

• Decidió autoprepararse para el examen sin clases particulares. — He decided to prepare himself for the exam without private lessons.
• Los atletas se autopreparan durante la temporada baja. — The athletes train themselves during the off-season.
• Es difícil autoprepararse para una entrevista tan importante. — It's hard to prepare yourself for such an important interview.

In everyday speech, prepararse alone already implies self-preparation in most contexts — autoprepararse is only worth the extra syllables when you want to specifically stress the absence of outside help.