First, let's make sure we're adding it with the overall correct procedure.
Make sure to bloom the gelatin in a small amount of water and make sure not to boil the water. How I typically do gelatin is add a little water to either a sauce pan or a ramekin and then heat over the stove or the microwave gently til it's 100 or so deg F. Sprinkle the gelatin in and then continue heating and stirring until clear. Kill the heat and let cool to room temp by placing the pan in a sink with water and changing it out every 10 minutes.
When I add, it's when I'm transferring, however we can also add it to an existing carboy.
Rule number one though is NO SHAKING! You'll oxygenate the beer.
You may need two people to pull this off. Use something to very gently stir the beer no more than halfway down the carboy. Then slowly add the gelatin mixture with a funnel, a baster, a syringe, or something else so long it's sanitized. I cannot stress enough the need for stirring gently, you just want to see the liquid moving, no eddys or bubbles.
Before you add it, take a sample and see how clear it looks. You may not even need it if it's been in a fridge for a few weeks.