Okay first I'll explain what soft resetting is.
Soft resetting involves pressing certain buttons to reload the game from the start-up screen.
For example say you wanted to catch, I don't know... a Shiny Ho-Oh in Pokémon Heart Gold. What you should do is approach Ho-Oh and save the game right in front of it. Then press A to engage Ho-Oh in a battle and if it is not Shiny you should soft reset your game.
You can soft reset by pressing the following button combinations:
DS Games: Press and hold L, R, Start, and Select.
3DS Games: Press and hold L, R, and Start or Select.
With that being said, let's move onto the Masuda Method.
The chance of hatching a Shiny Pokémon from an egg via the Masuda Method is 1 in 1638 in Generation IV, 1 in 1365 in Generation V and 1 in 1024 in Generation VI.
Soft resetting 900 times does not guarantee that you will get a Shiny Pokémon. We'll use the earlier example of trying to get a Shiny Ho-Oh. I could be soft resetting to try and get Ho-Oh Shiny and it could only take me 50 times soft resetting to get it. On the other hand it could take me over a 1000 times soft resetting to get it. I'm not saying that it takes you between 50-1000 soft resets, I'm simply saying it all depends on luck as some players have got it after they're first soft reset, while others after thousands of soft resets. But obviously the more times you do it the higher the chance you have of obtaining a Shiny Pokémon compared to if you only did it a few times and give up.
This is the same when soft resetting to get a Shiny Pokémon via the Masuda Method. To soft reset to get a Shiny Pokémon via the Masuda Method, you should save before the Day Care Man gives you, your egg not after. As it is determined whether or not the Pokémon will be Shiny when it hatches from the egg, when the egg is given to you. When you receive your egg run around until it hatches if it is not Shiny soft reset the game.
Words of Wisdom:
When soft resetting you can easily get bored, tired and frustrated so it is best to be patient and remain focused. I once soft resetted until I got a Shiny Pokémon but I wasn't paying much attention and I was in the routine of pressing the buttons that I soft resetted on the Shiny Pokémon.
This concludes Dan's walkthrough on the Masuda Method, getting a Shiny Pokémon and soft resetting.
Hope this helped :)