It is very simple. Take a quart size baggie, fill it with water from the tank itself, secure the baggie to the side of aquarium with a couple of clothes pins or a potato chip bag clip.Make sure to keep the top of the baggie open and above the level of the aquarium water, on the inside of the aquarium, so that all that separates the fish is the thin plastic baggie. Place the betta in the baggie.
Now as the rest of the fish get curious and investigate you can see if the betta will greet them aggressively or not. Please understand that Bettas or Siamese Fighting fish are very aggressive and attack almost everything. Do not put the betta with another male betta as they will kill each other. Do not introduce Cichlids to your tank.
The betta can remain in the baggie sevral days and , does not need the oxygen pump to breath.