I'll give you a bit of a breakdown
@Zeekwitt which may help you out.
Anatomy
The entrance of the vagina is usually 1-.5 inches above there vulva "fortune cookie." You when you go in it's important to also push your dick up. So I would advise sticking the head of your penis into the vulva and then raising your penis up by in inch before pushing in. (Some people have also had luck entering at a 45 degree angle, as well) That's just for entry alone. You will also want to make sure she an take two finger. (If I recall correctly you're slightly on the think side when it comes to dick size, do three fingers may be necessary.) Make sure you put lube on your middle finger and finger her vagina. I usually use one glob the size of 1.5 peas plus making my girl cum twice by hand. You may have to use two pea sized globs of lube. Put one glob on your finger, finger fuck her. Put another glob on and finger and finger fuck her again. You will also want to make sure that your penis is lubed and also quite wet/slick.
Stretching (If needed)
(PLEASE NOTE: Always cut your nails as short as possible, and always thoroughly wash your hands before sexual stretching. Lube will be your friend, but make sure it's water-based only, with water as the first ingredient. e.g. Astroglide lube,
Link here!)
For most girl's your girl's size, generally there will be a "ring of muscle" about 4-5 inches from the entrance of the vulva. With your fingers increasing in number as you move forwards, you will want to poke the ring (if possible, it might not be), but if the ring is resisting penetration (if you can reach it) it's better to stretch the muscle out, just before the ring itself.
Some girls will take a week or two to be stretched out, some will take a month. And if you cannot make any progress past one finger, then she'll likely be too small no matter how much you try stretching. This will most likely not be a concern with your girl due to her breed and size, but it's not impossible.
If you do get to a level of stretching where you feel like you can get your penis in, take things very slowly. Gently slide yourself in with a good amount of lube. If she feels too tight still, or most importantly, she looks uncomfortable, stop immediately! More stretching is needed. Always listen to her body language on these matters. You want sex to be mutual pleasure. Both parties needs to be in on this type of fun.
Starting off with small dildos and moving them up in size until you can comfortably get one in that matches your size is more effective. But I highly dissuade newbies and beginners from using them as it's easy to cause pain and mis-aim if you don't know what you're doing. This will cause the doggy girl a tremendous amount of pain. It's better to start with fingers when starting out.
Positions
If she is comfortable with missionary, this will make things a lot easier. But it's possible that you girl is like mine and only like doggy style. I'm going to assume that your dog is about the same size as my dog. My girl is a pure bred Newfoundland and is about 115lbs. I did start out by crouch-doggy-style fucking her, but it was pretty difficult and exhausting. The easiest way I've found to fuck her is to get her up on my bed first. My bed is just a mattress and a box spring, you could also try a couch. Once my girl is up on my bed I can actually enter her standing straight up. (I personally have to also stand on a rolled up rug on the floor to make sure our crotches align properly.) To align properly, holding your dick out straight it should align with her asshole. You'll just need to crouch ever so slightly when entering her. If she's large enough to have sex already, then it's probably going to take some trial and error, but if you keep working at it, you should eventually get the hang of it. But again, make sure her vagina is lubed and also your penis is lubed.
Cleanliness
Vaginas are not like assholes. You want to make sure you finger her with well watched hands, and you want to make sure you have sex with her with a well washed dick. I normally hop in the shower and thoroughly clean my hands and penis before I have sex with my girl. If you use soap, make sure you thoroughly rinse it off. If you do not rinse it well enough left over soap will irritated the lining of her vagina.
That's all the advice I can give at the moment, if I think of more things I will edit my comments and add them.
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