One of the missions you will need to do to prove your worth to Carlos in Far Cry 6 is freeing someone from prison. The mission is called Second Son and takes place in a school that has been converted to a prison. The good news is that this means the layout is quite basic.
You will start crouching behind some crates, so drop back a little and then move up the stairs to the left. Follow the path around, turning left again when you can, then go right. Straight ahead you will see a guard and a security camera. You can take them both out with a silenced weapon.
The building to your right will contain Miguel, the man you are here to save. He will tell you that you need a key for the door. The key will be kept in the guard room. To reach it, you need to head to the next junction and turn right, then left, moving up the nearby stairs to the back of the building.
You can find the key in this room on a desk, and can then head back to Miquel and let him out. It is worthwhile being quiet for most of the mission, but Miquel is gonna be pretty rowdy on the way out, almost certain starting a fire fight. You will need to lead him through the small front doors at the front of the compound to get him out.