HI Rachel!

I am not as familiar with WWE, but I am very familiar with PHFHG, having taught it once years ago to my oldest, and now again to my middle ds. I would be glad to try to help here, and I think the best way I could do so is to describe the writing in PHFHG's plans. Here is a list of the writing students do in PHFHG...
R & S Grammar (I have my ds write about 5 answers, and we do the rest orally)
Dictation (3 x a week)
DITHOR Student Book (if you do Level 4/5, approximately 3 workbook pages a week, with more writing on a page; if you do Level 2/3, about 2 workbook pages a week, with less writing on a page)
Science Notebooking (2 times a week, approx. 4 sentences)
History Notebooking (2 times a week, approx. 4 sentences)
Science lab report (approx. 3-4 sentences - one for the "Question", one for the "Guess", and one to two for the "Conclusion")
3-5 vocab. cards for history (1 time a week)
Common Place book (copy Bible verse, quotations, special things from reading, probably 3-5 sentences, 1-2 times a week)
Creative Writing of a poem using RLS's poetry as a model (approx. 2-4 stanzas, 1 time per week)
Written Narration with history (5 sentences, 1 x a week)
Science short answer (answer 5 questions, 1 x a week)
I think it would be helpful to teach the vocabulary assignment before beginning PHFHG. You can just have your dc do 1 card to learn the process, and you can use PHFHG's first week of plans (it's on Day 2) to parallel the teaching of it...
http://www.heartofdakota.com/pdf/PHFHG-sample-week.pdf
I also think it would be very nice to begin DITHOR earlier, just 3 days a week, and work through at least 1 genre before starting your formal school year with PHFHG. Each of the genres follow a pattern, so having 1 or 2 units completed would be great because you can just focus on the rest of PHFHG when you begin that (plus, DITHOR is awesome).

You could also begin dictation (using the PHFHG manual), to familiarize dc with that, if you would like, but this is less of a priority to start earlier than the vocabulary and DITHOR.
Written Narrations are introduced in PHFHG in such a clear way, that we've had great success with starting written narrations off on the right foot. For this reason, I would not try to begin teaching written narrations prior to starting PHFHG, as you will have all the "helps" needed to help your dc will need to do well.

The creative writing is also introduced in a very clear way, along with the RLS poetry, that I would not try to parallel that by starting something similar earlier. The 5 question science short answers are also introduced in PHFHG, so need to begin that earlier either. I know you didn't ask this, but it would also be good to practice oral narrations, as PHFHG is assuming dc have had experience with that (not sure if that is something your dc have been doing or not).
For notebooking, it is nice if dc have had some experience with putting together a notebooking page, following 3-4 sequenced directions. You can see examples of how this is done in the first week of plans, but again, this is done so clearly in the plans, I'm not sure it's necessary to start it any earlier. I guess I would just want to be sure my dc had worked their way up to being able to handle the amount of writing in PHFHG, so making sure your dc have the fine motor skills to be able to complete the physical act of the writing would be good to train for too.
I hope something here helps. We have loved PHFHG so much in our home. We have so many favorites in PHFHG - it just is full of great books, benchmark learning, neat discussions, fun hands-on activities, deeper BIble studies - well, we love it. HTH!
In Christ,
Julie