Generally you cannot use nets. Jellyfish being delicate, and mostly composed of water, they will often fall apart in nets. They may get cut on the net and form a hatch like look or crush under their own weight. To catch a jellyfish I recommend doing one of two things. You can either scoop they jelly up in a bucket or let it swim into a bag underwater. The idea is to keep the jellyfish suspended in water at all times. Remember to record the salinity, water quality, temperature etc. This will help increase the survival rate of your jellyfish massively.
Now you know how to properly catch a jellyfish, but where does one find jellyfish? Good places for catching jellyfish are calm waters and eddies. Jellyfish are completely hopeless against the tides and currents. They can easily be pushed into calm waters. Once there, they cant easily leave. Long, low lying docks, drop offs into deep water, very calm beaches etc are good jelly spots Jellyfish can also often be found in tides and waves on the beaches. This isn't a good place to find healthy samples. The jellyfish sound here, if alive, will often have injuries and may die very shortly.
Even if you check all of the good spots in a location you may still end up with zero jellyfish. This is because jellies tend to travel in huge clouds or "smacks". They can be very random and hard to find but once you do, you may be faced with literally millions of jellyfish. Cannonball jellies have been seen in smacks of thousands and even millions.
Once you have your jellyfish, your going to need to get it home. for people who live on or close to beaches, there is no problem. But many people live a good distance from the ocean. To maximize their chances of survival I recommend bagging the jellies in large bags with a lot of water. If you can, replace the air in the bag with pure oxygen gas. Try to keep the jellyfish in each bag to a minimum, to prevent build up of waste. Bigger jellies may need to be placed 1 per bag. On the contrary, you can bag several small moon together.
Taking any living organism from an environment leads a high risk of harming that environment. Luckily, jellyfish almost always occur in huge populations. That being said, many jellyfish supply food to many organisms. In my case the cannonball jellyfish is a huge source of food for the endangered leatherback sea turtle. Cannonball jellies do happen to make up 16% of the Atlantic Ocean's biomass. I still refrain from gathering large amounts of cannonball jellies from one location to prevent a harm to the environment. Try to take just as many as it takes to obtain a male and female jellyfish. Then polyps can be produced, leaving you with infinite jellies.
Also, never dump jellyfish ,or even water that jellyfish have lived in, into salt water. Microscopic jellyfish stages could end up introducing a new species. This has occurred before, and has decimated populations of other organisms.