This is a tutorial my daughter and I did back in 2009. I put it back up to answer Bowsnbucks question about collecting urine (on another thread).
We are using a red fox for this demonstration.
Start with the skinned carcass still hanging on the gambrel.
Make an incision on the center of the belly. Be careful, if the bladder is full it will be just under the surface, make a very shallow cut.
Spread the incision open and pull the hopefully full bladder outside the body cavity.
This is what an empty bladder looks like.
We use a plastic pint bottle and a small plastic funnel. With plastic you don't have to worry about dulling your knife on metal or contaminating urine with the rust that will form on a metal one.
Place the bladder (still attached and intact) in the funnel.
Then stick it with your knife.
It is the fastest, cleanest way we have found to remove urine from caught fox.