Firstly you need two things - Paint and a brush. The paint I'll be using is Tamiya Clear Red and VGC Black.
The following thing you require is a brush. I stipple it on, so I have an old brush which I cut the bristles right down.
The first thing you'll want to do is add some black to the red. Be careful here, the black is strong and will overpower the red, making it pretty much black. You want it to be a browny, redy, black mix. This picture it is hard to see, as it looks very black. Also, the clear red is very thick, it'll need quite a bit of watering down.
So mix this up, then get some on your brush. You don't need much, wiping some excess off is a good idea. The plan is to stipple - or to 'stab' the brush onto the surface, rather than stroking it as you normally would. The red mix will be dark and dries quite dark. The first layer looked like so:
Let it dry and continue until it looks dark enough. If you're confident you can stipple or paint some splatter a little bit further away from the source. This, when dry will look like dark, dried blood.
The final layer, if you want to do this, is to stipple on some pure Clear Red. This is strong and will give the look of fresh blood, so apply it to the edge and try to leave some of the darker, dried blood showing.
That, when dry, should give you a nice bloody look. I hope you enjoy this.