How should a person who follows the imam make their intention?

Intention (niyyah) is one of the prerequisites of prayer. A person must determine in their intention which of the obligatory, necessary (wajib), or supererogatory (nafl) prayers they are performing, which prayer time it is, and whether they are praying alone or following an imam. What matters is that these are known in the heart; verbally expressing the intention is considered commendable as it reinforces one’s resolve in this matter. (Merghinani, al-Hidaya, 1/46)

Accordingly, a person who will perform prayer by following an imam must make this intention in their heart; otherwise, the prayer performed without intending to follow the imam will be invalid. Additionally, verbally saying “I follow the present imam” is also appropriate.

Source: T.C. Diyanet İşleri Başkanlığı, Din İşleri Yüksek Kurulu