Think about the angle
No matter what you're splattering fake blood for, whether for a short film, or a costume, or a really mean prank, to get the full effect, you need to take into account the angle of the splatter and from what part of the body the blood came from.
Make sure you do your research. If the blood came from a main artery, it would likely spray quite a bit. If the victim was on the floor when they started bleeding, the spray would be closer to the ground and in the direction the wound was facing. If it's a smaller wound, and a person is dripping blood, a cut higher up on the body (or even just from a taller person) would have more splatter when it hits the ground than on a lower cut, because it's falling from a greater distance.
Get creative
Think about the final product that you want and see what tools you have at your disposal to get the desired result.
For a very fine splatter, try dipping a clean toothbrush into fake blood and flicking it by running your thumb against the bristles.
For a splatter with slightly larger drops, dip a paintbrush in fake blood and "whip" it where you want the splatter to go.
For a large splatter, try soaking a rag or old shirt in fake blood and again "whipping" it where you want it to go. You can use any number of tools, the best part about DIY is that there are no rules. The only limit is your imagination. If something is stupid but it works, it's not stupid.
