To Envelope or Envelop a Letter?

If I’m enveloping a letter, what do I use?

You do not use "to envelop."

OED:

2. To wrap, cover closely on all sides with a surrounding medium (e.g. clouds, darkness, flames, an atmosphere, etc.). Const. in, with. Also with the surrounding medium as subject.

1847 L. Hunt Jar of Honey (1848) ii. 15 One of these cliffs towers to such a height, that its summit is for ever enveloped in clouds.

The subject of the verb to envelop does the action:

"A dark mist enveloped the castle." - it is the dark mist that does the action of enveloping.

In "I envelop the letter." this means that you yourself would surround the letter on all sides or wrap yourself around the letter.

I suggest,

I will “Write a/the letter, put it in an envelope, and post it”.

However, it is hardly worth adding "put it in an envelope" as this would naturally be assumed because nobody sends open letters in the post.