Very, very hard. Take yesterday.
I needed editing supplies, which meant getting three pocket folders, post-its, subject dividers, and a ream of paper. So I needed to get all of that, and get it back home without my roomates - who I was shopping with - noticing that I'd bought it all. A pretty hard task, right? It involved splitting up with everybody, running from Ryman Stationary to Tescos and back, and carrying around a 500 sheet pack of paper for two hours, as well as everything else. I literally had cuts on my hands from carrying the heavy plastic bag around, because the handle kept cutting into my hands. Ow.
I printed off a lot of chapters for my editing work, and though I've printed off 100+ pages, it's only over a quater of the actual novel. I had to print all the chapters while said roomates were out or busy, so they didn't stumble onto me printing lots of pages of writing. And now, we don't have much paper. So to keep suspicion down - I hardly ever print anything - I can't bring up the fact that there is low paper, or low ink. Now, I've got to wait for somebody else to discover this and buy a new cartridge before printing new chapters. I would buy the ink myself, but not only do I have no idea how to put it in the printer, but after a big London shopping trip, editing supplies and new haircut, I have no money.
Keeping the secret is hard, expensive work. But I hold onto the hope that it will all be worth it in the end. If seeing the finished product is anywhere near as emotional as finishing a first draft, I'll deifnitely have a good return.