There are a ton of different Fast Castle build orders. We are going to show the basic 'Fast Castle knights' first. Then additional in depth advice will be given. Without further do here is the build order:
Best way to practice Fast Castle?
There are specific mods to practice Fast Castle. You can find them here:
If you don't like practicing in mods you can just go to multiplayer and check your castle times.
What are good castle times?
You have to check the in-game time, not the real time. Good castle times are around the 15 min mark. You can go faster if you try going up at 25 or less vills but then you will sacrifice either economy or military.
When to go Fast Castle
When you are the 'Pocket player'. This means your position in the map is away from the opponents (you're in the middle of your allies). You can also go Fast Castle in maps like Black Forest if you wall correctly or in other maps if you're good at defending.
Generally go Fast Castle if you're new to the game. This is the first build order you should learn and master. If you do it right it will ensure you 1650+ elo in steam.
When not to go Fast Castle
Generally when playing 1v1 against ~1700 (steam) elo players. They will generally go Archer Rush, Tower Rush or Scout Rush, which counters Fast Castle unless you are really good at fast castling,defending and walling.
In team play don't go Fast Castle if you're playing an open map and your initial position is near your opponents.
What to do after Fast Castle
There are several options. The most common is making 2 knights, send them to attack and then do more knights to keep attacking. You can also follow one of this options: