First you sneak up behind the egg while it's carrying soup and yell SURPRISE!

Seriously the trick is have the stock slightly thickened with corn starts. Only LIGHTLY beat the eggs don't whip them. You should have some yolk bits in there. Then stir the very rapidly with one hand and drizzle in the egg---I use a small soup bowl with a couple of chopstick I can hold with my thumb so it drizzle into falling in in one clump. stiring rapidly is a key...sometimes it helps to have extra pare of hands to stir while adding the eggs.

Start with very hot stock, and add egg in a very thin stream and slowly. That will result in tendrils of cooked egg in your soup, rather than clumps

Here's a nice tutorial from The Kitchn: http://www.thekitchn.com/how-to-make-egg-159379