Ok, this one takes a bit of artistic talent.
In asda they sell fake "scabs" and things.... right what you need is a few of these! Draw a "slit" accross your throat with back eyepencil, making it wider in the middle and decorate with the scabs - making sure to cut them to shape. A bit of tit tape will hold them in place very well. Then - best bit - old strappy top (black) with any trousers (black is aways good) and literally POUR fake blood down your chest from your "wound".
It works amazingly, it's sticky, but i have won so many dressing up sompetitions with this. Washes right off in the shower too. Effect ony works if you literay POUR the "blood" from your "slit" all the way down your chest till your strappy top covers all the "non-bloody" skin.
It sounds so complicated - but its great.
A tiny bit of white face paint and some very dark eye make-up helps too. Big tip tho, your lips cant be bright- use a little bit of lip balm on them to lighten them.
Good luck xx