Separate the signal from decoration
Start by listing only observable features: shape, count, position, direction, size, fill and border. A decorative detail can distract you, while a small repeated change may be the real rule.
Compare adjacent frames one feature at a time. If rotation changes but count and fill stay fixed, test a rotation rule before combining several explanations.
Use the simplest rule that explains every step
A good answer must fit the whole sequence, not just the final pair. Reject a rule when it requires an exception that the picture does not support.
When two rules look possible, inspect direction and step size. A triangle rotating 90 degrees follows a different pattern from one alternating left and right.
Check the answer before committing
Mentally apply your rule to the last known frame and predict the missing frame before viewing the options closely. This reduces the chance that an attractive distractor will steer your reasoning.
Then eliminate options with one impossible feature. This is often faster and more reliable than trying to prove every option from scratch.