So you're stuck on this quest, huh? What's your issue?
How do I use these putrefying bolts?
Open your inventory, drag the bolts to the top circle, left of your character's equipment. It should then appear on the bottom of the screen. You can then use it with the Q key or by clicking it.
How do I get the body tissue from the rathound?
Shoot a rathound with the putrefying bolt, then wait a bit. The bolt will not kill it outright, but it does have to seep into the tissue. Give it ten turns to be safe. Just find a fence, keep your rathound buddy on the other side, and wait. After that, kill it as you please. It should yield remains with the body tissue. Worked for me anyway.
I ran out of putrefying bolts. What do I do?
Load the game. If there are more of these bolts, I can't find them.