The grid makes it so, so much easier. Hardcore cross stitchers look at it as a crutch, but eh, do what you gotta do
And yeah, buy the EZ guideline stuff- it's totally worth it, and a spool lasts a long time. Much better than stressing over little bits of red thread.
And I thought of a few more things:
Usually you're supposed to start stitching from the center of the pattern, but with big things, it makes more sense to be to start from the corner. Just make sure you center the pattern on the fabric so you don't run out!
And since you'll be working on the same piece of fabric for a long time, make sure your hands are clean when you stitch or else you'll have super grimey aida by the end. I think most people have to wash the finished piece anyway.
Lastly, more stuff to buy, but also inexpensive: Q-snaps are so much better than hoops. They don't crease the fabric as much and you can get bigger ones. Best thing before actually buying a scroll frame (which I eventually did, but costs a whole lot more). http://www.better-cross-stitch-patterns.com/q-snap-frame.html
Good luck and come join us on the big project thread soon!