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The best recognized method to get flying squirrels out of your attic is to use an exclusion-based method. You are encouraging the creature to find a new home of its own accord, without intervening too much, all by waiting for the creature to live and then locking right up after it. When the door is locked, the animal can't get back in. The exclusion way is a wild animal removal process that can work for a wide range of other critters too, including bats.
If you decide NOT to use an exclusion method, instead opting for a live cage trap approach, you should know that you are going to need to drive quite some distance before you can release the flying squirrel again. It must be at least five miles from your home, although we would recommend ten miles or more. If you don't go far enough away before you release it, it will be able to find its way back to your land again in almost no time at all.
Secondary to this, you will very rarely find just the one flying squirrel on its own in your attic. It will more often than not be a mother flying squirrel, either pregnant and about to give birth or taking care of her young in already, and although the fathers don't stick around for very long, the women do love to stick together. You will commonly find a large number of flying squirrels living together, all helping to take care of the youngsters. One trap is certainly not going to be enough to get rid of the entire group of flying squirrels, and even if you were to set a couple of traps, there's a good chance there would still be a couple of the smarter mothers still hiding away in your building somewhere.
Thirdly, you will need to do your research before you decide to trap, either lethally or otherwise, flying squirrels. In most states, you're not actually permitted to trap and release flying squirrels, or harm them in any way, and there is even one species that is endangered. The last thing you would want to do is endanger an already endangered species, especially when it's a helpful one, like the flying squirrel. As bad as it can be for destroying flowers and plants, the animal is also prey for eagles, owls, coyotes, and other predators, and the squirrel itself preys on insects and other things that you would rather not have more numbers of. Each animal has their own place in the ecosystem, and that's why exclusion methods are the best option. Intervening too much will almost certainly induce some sort of harm on the creature, whether that's in the form of actual, physical injury, stress, or even death.
You could look at repellents and deterrents to keep the creatures away, but unless you go through the process of cleaning up after the animal, and then sealing up all the holes that it/they used to gain access, it will all have been for nothing. There is no magic fix for a nuisance wildlife problem, sadly.
We would always advise that you seek advice from a professional nuisance wildlife expert before you take any action, if only to find out the local laws and regulations if nothing else. Also, in many cases, it actually works out much cheaper to hire in the professionals than to try and attempt this job yourself. Many home and property owners waste hundreds of dollars, sometimes thousands, on failed removal attempts first.
For more information, you may want to click on one of these guides that I wrote:
How much does squirrel removal cost? - get the lowdown on prices.
How to get rid of flying squirrels - my main squirrel removal info guide.
Example squirrel trapping photographs - get do-it-yourself ideas.
Squirrel job blog - learn from great examples of squirrel jobs I've done.
Squirrels in the attic - what to do to solve the problem.