Quand je l'ai traduit en anglais, il a dit "That's how I got into the habit of taking the bus".

That;s exactly how I would translate "C'est ainsi que" in english

Quand j'ai traduit "Ainsi" tout seul, il m'a donné le traduction 'thus' (en englais).

You can't really translate "Ainsi" to one word in English. It's a connection word that is used in many different idioms in French with slightly different meanings.
Here are a few
- Si tout s'est passé ainsi, le doute n'est plus permis. == If this is how it happened, there can be no doubt
- Comme un naufragé s'agrippe à une épave, ainsi il se raccrochait à ce faible espoir. == As a shipwrecked man clings to a wreck, so he clung to that faint hope
- Ainsi donc, tu as changé d'avis ? == So you changed your mind?

As you can see, ainsi is translated to many different group of words. It's a word that makes a reference to a process or an action, but can also be used in idioms that are to be taken as is.