Get a reasonably fat book that you never plan on reading again. I used a book that I found laying around the house called "Redwall" by Brian Jacques. Except it's in German... sooo... yeah. Mark a square/rectangular shape that's a reasonable size to fit a ziploc bag of blood in. Next, using an Exacto knife, cut out the square an inch or so deep. This will take a while, so be patient.
Step 2: Fake Bloooood!
Make some fake blood that's not too runny, but not sticky either. http://www.wikihow.com/Make-Fake-Blood <--- this site has a bunch of different recipes. I recommend using either the corn syrup one or "Realistic Looking Edible Blood" Once you have some of this (No Exact Measurements, just eyeball it :) Now, pour this into the baggie, and close the ziploc for now. When you're ready, just open the ziploc a bit, put it in the hole that you made in the book, and squeeze!