The best way to learn proper English is to read news report, and watch news on TV. Watching TV shows is a great way to learn casual English, slang words, understand culture reference and humor. If you have already watched these shows then you may recall the words used in the following dialogs.

It is CUSTOMARY to kneel
when surrendering to a king.
Game of Thrones Season 4, Episode 10

I know it's CUSTOMARY
for the birthday boy to receive presents,
The Big Bang Theory Season 9, Episode 17

...in order to indulge in your CUSTOMARY
preemptive pre-show urination.
The Big Bang Theory Season 3, Episode 5

It's CUSTOMARY, when
using the restroom
The Big Bang Theory Season 5, Episode 13

It was CUSTOMARY for me
to extend him an invitation.
Game of Thrones Season 2, Episode 5