Correct spelling, explanation: proper is an adjective and an adverb. It has to be spelled with a single -p consonant because its etymology indicates that. This word comes from Old French propre and from Latin proprius. There’s no reason why someone should write it down with two -p letters. If you’re wondering what a specific word should look like, always check its history and you’ll find out.

Definition of proper:
1. adjective, strictly belonging or applicable
It’s the proper place for a stove.
Remember to put your documents in the proper place.
2. adverb, thoroughly
Wash the fruit properly before eating.
Meat must be properly cooked.

Collocations with proper:
Some most commonly used collocations include: a proper goodbye and a proper time.
Give your grandfather a proper goodbye before he leaves.
It’s a proper time to go on a date.

Incorrect spelling, explanation: many words in English come from other languages. It is no different in the case of the word proper, which originates from Old French and Latin. If you want to know how to spell it correctly, just look at the etymology. We can see that it contains only one -p letter. It would be a mistake to spell it with a double -p consonant. Propper is not correct.