I posted this in another thread on another board, about how to use an inflatable dog dildo and some other stuff to best simulate it. It can at least give you an idea.
Now, if you want to properly emulate what it's actually like, you'll need to following items, and preferably a friend to help you out.
A roll of carpet, about 100 pounds worth, of whichever type of shag you prefer; I like shorthaired dogs.
A run-of-the-mill water pistol, of a capacity at least a cup or so.
Lots and lots of lube.
The dildo, of course.
Now, what you're going to want to do is thin the lube until it's very watery and heat it up to a little over 100 degrees, and also microwave the dildo or otherwise heat it to the same temperature while you're at it. Next, liberally coat the dildo with the lube and save about a cup for the water pistol. This next part is what helps to have a friend: get naked and on your hands and knees and right away drop the entire roll of carpet on your back and simultaneously squirt basically the entire contents of the water pistol onto your hole of choice, not caring if most of it misses the mark. Without waiting for more than a couple seconds, jam the dildo inside yourself, hilting it fully in 3-5 thrusts, and immediately begin inflating the knot. Continue to yank back and forth on the dildo for about 30 seconds while the knot inflates inside you to baseball-sized. Oh, I forgot, you're going to need a hairdryer blowing on the back of your neck while this is happening. I'm not sure how you could emulate the wetness, but try to pulse the thing on and off rapidly between thrusts.
When all this is over, depending on how well-behaved you want to imagine your dog is you can either leave the dildo inside of you for 20-45 minutes with the carpet on top, or kick the roll of carpet off your back and behind you, and pull really hard on the knot for a roughly equal amount of time.