The first step in catching a fly is to take your dominant hand and bend it into a cup shape.  Practice quickly shutting your fingertips against the heel of your hand. Make sure to leave a hollow space inside, as this is where you will trap the fly. Be careful. If you shut your hand too tightly, or make a fist, you will simply crush the fly. If you don't mind that the fly dies, then this won't be a problem. When catching a fly with your bare hands, it's best to wait until the fly lands on a flat surface, such as a table or counter top.  Move slowly towards the fly. Any sudden movements can set the fly off and you will have to wait for it to land again. Waiting until the fly lands on a stable surface makes it easier to predict the fly's movements. Make sure the surface is clutter-free, as you don't want to knock anything over in your fly-catching attempts. Once the fly lands, take your cupped hand and wave it a couple of inches above the fly, closing it, just like you practiced, as you wave your hand.  As the fly senses your hand movement, it will get spooked and fly straight upwards and directly into your cupped hand.  As soon as it enters your hand, close your hand quickly to trap the fly inside. Now you are free to release the fly outdoors, place it into a jar for your inspection, or feed it to a pet.

How would you rephrase that briefly using Vietnamese?
Khum bàn tay lại. Chờ cho con ruồi đậu xuống. Khua bàn tay đang khum bên trên con ruồi.