First things first, you'll need reference images.
Also Lab604 made an incredible Big Sister and documented every thing on flickr.
How I made my Syringe.
I started with a piece of Styrofoam carved into shape and covered with filler. Details were added using foam mat and hot glue.
The end product was painted brass and copper and then weathered.
One of the valves were bought on Etsy.com and spray painted red, the other was just made out of foam. Both were just screwed right onto the Styrofoam body and can be twisted.
The needle is just a long pvc pipe, and it is possible to detach it. I used the mechanism of a small shampoo bottle for this.
The needle tip should have been angled and the blood a little darker ;)
I have used a piece of a soda bottle as part of the arm guard (see picture below). And a faux leather belt to attach it securly to my arm.
Strap it on and you are ready. On a different note, I will make a Little Sister Syringe very soon ;)
Photo: Annah Lång
And since I could take off the needle I had to use it as a cane for my dance runtine in 2012. I could not see a thing, but it is still a little funny :p