I am reluctant to answer on such an old post, I seldom take joy in raising things of this nature from the dead; however, I stumbled upon it and I suppose it's still relevant.

A lot of words are passable for this scenario: Input, Enter, Fill, etc.

"Input user credentials" "Enter username and password" "Fill in the required text fields" "Complete the form to proceed"

They all work and in terms of "best" it's irrelevant as stated in the accepted answer.

That said, as of late, I've been documenting restrictive states instead of trying to offer direction, e.g. "The username text field is required"

I feel this is more concise and direct. Instead of attempting to give the user/reader direction as to how to proceed, inform them of the limitations they may encounter and let them sort the "how."

I am reluctant to answer on such an old post, I seldom take joy in raising things of this nature from the dead; however, I stumbled upon it and I suppose it's still relevant.

A lot of words are passable for this scenario: input, enter, fill, insert, etc.

"Input user credentials" "Enter username and password" "Fill in the required text fields" "Complete the form to proceed" "Insert thyne response via your preferred input apparatus to unlock the eloquent mysteries of the number 42"

They all work and in terms of "best" it's irrelevant as stated in the accepted answer.

That said, as of late, I've been documenting restrictive states instead of trying to offer direction, e.g. "The username text field is required"

I feel this is more concise and direct. Instead of attempting to give the user/reader direction as to how to proceed, inform them of the limitations they may encounter and let them sort the "how."