If you to say, "I want to fuck my full-grown adult Labrador retriever in the ass with my average-sized penis, but how do I make it fit?" then I could answer you a lot more easily. That is actually a reasonable question.
For all that I know, you are attempting to fuck a chiwowow dog or a Shih Tzu dog, and that would be rather dangerous, actually.
I can tell you, as a gay man, that having things shoved into your rectum when you are not expecting it hurts. Anal rape was used as a means of punishing prisoners and slaves for centuries. The reason why was that it not only humiliates them, but it also hurts like a motherfucker if it's not done right and with a sense of empathy for the partner that is being penetrated.
It works better if you get the dog to push back on whatever you are trying to put in there. If you give the dog a history of positive experiences with things being shoved into their butthole, starting out by being very gentle and never forcing them to let you linger longer than they want you to, then eventually, the dog might actually start poking their buttholes back as if they were trying to swallow your finger.
It helps build positive experiences if you have your finger covered in lots of lubricant. Lots and lots and lots of lubricant. The statement, "fucked without lube," comes from the fact that fucking someone without lube is just plain mean.
I am not saying that an experienced total bottom couldn't take it like a champ: they know how to relax their sphincters at the drop of a hat, and because they have done it so often, in some cases practically for their bread and butter, they can go straight from being fully clothed to having a boner up their butts while successfully pretending it's just a PG-13 lap-dance, no lube, no inching it slowly in there, no "owwwwwwww, I didn't expect it to feel that way! I think I broke a nail!" but completely casually and without build-up. They can get away with that because they have been doing it for years. They take yoga classes so that they can learn how to relax their muscles on demand. You know that a lap-dance is more than a lap-dance when the person doing it clearly knows exactly what he is doing.
Someone that is not used to having things shoved up their butts needs to be introduced gradually to the fact that it's not the end of the world to get fucked, but on the contrary, if you have a sense that you are in control of it and know what you are doing, it actually makes you feel like a god. It feels amazing. It's like finding out that the world is really flat, after all, and you suddenly realize the amazing truth: Aslan is gay, and he likes to reward his subjects by letting them fuck his majestic butt. Once you have spent a while feeling like a god because of the fact that getting butt-fucked properly, when you are experienced and mostly in control, feels so intensely amazing, you cannot ever think about a god ever again without wondering if he likes getting stuffed like a turkey for fun. Ever notice how that lion walks with a sashay?
If you are going to build up your dog's confidence that getting fucked in the ass can be a good thing, then start with slathering your fingers with lube, and start giving puppers very gentle massages in there. Give puppers a chance to learn how to ride your finger like a dildo. If puppers has even the slightest predisposition for liking to get buggered, then they'll keep on coming back for it, once they understand it.