I would go to a vet or groomer for this, however what you need to do is stick your pointer finger into the dogs rectum and on each side of the buttocks squeeze where it is swollen.
flushing is not the same thing as expressing the glands. the answers above are not about flushing. flushing is a surgery done by vet under sedation. they put a tool in there to flush it out with water.
The best advice anyone can give you for that is to call your vet if you dont do it right you can seriously injure your dog and I know you dont want to do that the vet may even show you how the first time.
