First, do not open the transport bags of your Discus before you are told.
When receiving your Discus, take the temperature of the water in the bags. Best with an infrared thermometer. If you only have a classic thermometer, wedge it between 2 bags to sandwich it at the water level.
If the difference in temperature between that of the bags and your aquarium is more than 4° Celcius read step 1, if less than 4° skip to step 2.

Step 1 :
If the temperature difference is more than 4° Celcius, fill a bucket of water with water from your aquarium which is at 28-30°.
Soak your bags in the bucket for 20-30 minutes.
If you have a lot of bags and a box of styrofoam (or cooler), leave your bags in the box and pour in some water from your aquarium.
The goal is to raise the temperature of the water in the bags to 24-26°, so that the difference decreases to 4°.
Take the temperature of the water in the bucket or box. The temperature will drop, this is normal. It goes down while the one in the bags goes up.
The goal is to raise the temperature in the bags slowly as this could cause thermal shock if the temperature rose too quickly to 30°.
In order not to raise the temperature too quickly, do not put too much water in your bucket or box, put about the same amount of water as there is in the sum of all your bags.
After 30-45 mins the water should be 24-26°, if not, add water from your aquarium to the bucket.
This step is finished when the gap between your aquarium and that of the bucket or box is 4°.

Step 2:
If the temperature difference is 4° Celcius or less, you can soak your bags directly in your aquarium which is at 28-30°
After 20-30 mins, open the bags and pour in a small glass of water from your aquarium every 5 mins so that the starting water volume is tripled or more in 30-45 mins maximum.
Afterwards, you can release your Discus in the aquarium without the water that is in the bag that you throw away.

Reduce or turn off the aquarium lighting the first few days until your Discus eat without fear.
It’s normal if it takes 3-4 days. In the meantime, keep them calm.