Is there a proper way to write a Proof of Employment letter?

If by "proof of employment" you mean a letter that states that you are employed, all that is required is for your employer to say so in writing. There is no magic formula. A simple form would be:

“This letter will confirm that [your name] is currently employed by [name of your employer].

The party requesting the letter may also want to know your position and length of employment, which can easily be accomplished by adding “in the position of _____, and has been employed since
_______.”