Step 1: Return the verb to the basic ‘he’ form if an ending has been added.
Step 2: Locate the middle root letter.
Step 3: Read its vowel:
● fatḥah → فَعَلَ: خَرَجَ
● kasrah → فَعِلَ: سَمِعَ
● ḍammah → فَعُلَ: قَرُبَ
For example, سَمِعْنَا returns to سَمِعَ, so its pattern is فَعِلَ.