In order to make a scary splattered shirt you will need:
A clean white shirt
1 package of red fabric dye
Latex / Rubber gloves
Small zippered plastic bags or water balloons
A large tarp
Step 1: Scarlet Dye
Add just enough boiling water (between 1/2 & 1 cup) to liquify the powder in your dye packet and mix.
Step 2: Set-Up
Bring your bowl of dye and other equipment outside and set up on the tarp.
Step 3: Get Messy
Use the straw, plastic bags or water balloons and your own hands to decorate the shirt.
Siphon dye through the straw from the bowl, cap with your thumb and let loose to create a long spray or dribble of blood. Fill and pop either plastic bags or water balloons for bold blood-bursts and use your hands to create prints or your fingers to flick smaller sprays.
To avoid bleed-through you can place a piece of cardboard inside the shirt or just get filthy, have fun & enjoy the bloodbath from the very centre of things (and shower immediately after).
Once you've covered your shirt in splatter, hang it up outside somewhere to dry. Do not wash.
Step 5: Clean Up
Deal with the grisly aftermath of your endeavour while you wait.
Scare your friends and strangers alike by stumbling by looking like the poster child for an anti-violence campaign. To keep the dye bright for your costume do not wash before wearing.