I shall share with you the recipe that I always use which I found out about online years ago (so I don't take credit for it)
-Golden syrup (or corn syrup if you're from the USA)
-Red food colouring
-Blue or Green food colouring
-Washing up liquid
1. Start by boiling some water in your kettle and while you're waiting for that to finish, pour the golden syrup so it fills ups 10% of whatever glass you're using.
2. Pour the boiled water into the glass and stir it until the syrup dissolves. If you want it to be thicker then add more syrup.
3. Measure a teaspoon of red food colouring and then stir it into your mixture. Then add a little bit of blue or green colouring to darken it. Put a teaspoon of coffee in there as well and stir until it vanishes.
4. At this stage I dip the tissue to see if it stains it the colour I want. Often I see videos on YouTube where the blood is too pink and looks fake. It's very hard to get it looking the way you want but eventually you'll develop your own formula that works best for you.
If you want the fake blood to wash out of clothes more easily then squirt some washing up liquid into your mixture. This will cause bubbles and I don't recommend you putting it in your actors' mouths so skip this part if you want "mouth blood".
I like to pour my mixture into an empty cola bottle and stick it in the fridge to keep it nice and fresh. However, clearly label it as fake blood unless you want a family member drinking a potent mix of detergent and instant coffee (true story).