it can be daunting
Tia hit the nail on the head when she said pick just one or two aspects to grade. Also a rubric will help.
When I grade a full-on writing process piece, I spend a long time on prewriting with a graphic organizer of some type. I walk around the room and give a grade for completion. You can tell how much effort goes into the activity and conference with students briefly as you make rounds. I usually give a checkmark-equals-100 grade for these if the kid puts effort into the prewriting process. I give 85 or 70 for lesser efforts.
The next step is 1st draft, and I emphasize the draft needs to be double-spaced and of a specific length (2 pages, or 3 paragraphs or whatever). Again I make rounds and give the same kind of grades for the effort.
Revising usually involves some mini-lessons on strong verbs, slotting, expanding, sentence combining, etc. I use overheads to teach the technique or a play or some other (hopefully) engaging approach to the minilesson and expect the students to make the revision strategy in at least 3 places. I usually teach/reteach 2-3 strategies per piece. I might make rounds again to have students show me their revisions, but not always at this stage.
Kids need to re-read the draft about now. I have some PVC tube-phones that kids use to read the drafts to themselves with a pencil in hand to make any changes.
The dreaded editing is next. I like to give students a check-off sheet to accomplish this. Scholastic publishes a book called Great Genre Writing Lessons that has some good check-off sheets. With a red pen, students must place a dot under every capital letter to ensure they capitalize correctly, circle 3 or more words with questionable spelling, draw a box around all end punctuation, etc. I might circulate and give a grade for editing. I just look to see that the paper has a lot of red on it. Getting a peer to read it is also effective to get the editing done.
By the time the kids write the final draft, I'm pretty familiar with all the pieces. If I get on the grading ASAP, it goes a lot faster than if I dread it and put it off and allow the papers to get cold. I have to admit I usually don't read whole papers--just the first paragraph or two and maybe the ending. I create rubrics using Rubistar on the internet.
Writing process papers takes at least 3 days for me. The sooner you start grading the better. It might help to stagger the classes so you only get 25 papers versus 100.