Buy a 5 gallon tank minimum, set up the filter and deco and cycle the tank. add an adjustable heater but dont turn it on.
Get your betta, put the cup in a plastic bag so you can float it in the tank without haveing the bag tip over, float for 20-30 minutes.
Add some tank water to a baggie, Gently dump the betta and cup water into a bag it should be about 50 50. new tank water and cup water.
Float 30-45 more minutes.
gently pour him into the tank. with all those fins, he could get stuck to the bag, you may want to add even more water to the bag before pouring, or cut the bag open so that he just falls out.
add stress cot to the tank
turn the heater on its lowest setting, dont raise it more than 2-3 degrees every 3-4 days. keep raising the temp until it is 78-82 degrees in the tank. Good Luck!
FYI, it is best to avoid useing a net with long finned fish, If you have to use a net, find a softer net or one for shrimp that doesnt have all those tiny holes in it. they are usually white. and always add stress coat product afterwords.