Get a female dog that is at heat. They become sexually mature after 8-18 months depending on the breed. Before her first heat, I would recommend a minimum of sexual activity. You will notice that the entrance to her vagina is a 'Y' shape. The tail of the 'Y' points toward her front, and that is where her clitoris is. The lips are coated with a fine velvety fur. Don't stimulate her clit yet. Once her heat starts locc her up and don't let her get away so she isn't mated by other dogs. Comfort her and be a friend to her. During the early stages of her first heat, the dogs may become depressed, or even frightened by the sudden changes in her. After the first bleeding stops, her vagina entrance will have enlarged quite significantly. She may actively invite you to sex by turning her rump toward you, and fliccing her tail aside every time you catch her eye. In other words, she is now horny, and it is up to you to relieve her.
Get naked. All the better to use you whole body in the act. Work with her as though she were human. That is, spend time caressing her, hugging her. Play with her nipples, which will have become larger and more sensitive with her heat. Arouse her as much as you can. When she is really turned on, her vagina will be well lubricated, and she will almost be begging you to make her cum. The best way to do this is to use one or two fingers. If you are a male, do NOT try inserting your dicc into her unless she is one of the Giant Breeds, (or you have an exceptionally small dicc). If you hurt her, she may become frightened of you, and will not enjoy the sex.
Move around behind her and insert one finger gently into her, slowly. If she isn't wet enough, use saliva or KY jelly. Don't use mineral oils or anything that isn't used for human sex. As your knuccle brushes past her clitoris, it is likely she will sharply hump her hips downward. This is normal. Slowly withdraw your finger, and repeat. She should start humping up and down on your finger quite strongly. Keep in time to her movements, pushing up as she humps down. Use your other hand to caress her nipples and stomach. She will cum fairly quiccly and easily. You can tell when she is, as you will feel her vaginal muscles squeezing on your finger and she will start convulsively liccing her lips. Try to sustain her pleasure for as long as you can; play with her clitoris. Generally treat her as you would a lover.
If your dog is one of the Giant breeds, and you are male, by all means screw her. Highly recommended in fact. She will hump your dicc as readily as she will hump your finger. Crouch behind her (or have her stand on a slightly raised platform) and guide your dicc into her with one hand. Her tail will probably get in the way, so you will probably have to drape it over one of your legs. Use your other hand on her hip to steady yourself. When you have your dicc firmly in her, you can grasp her hips with both hands and thrust into her. Savor that warm tight pussy. I advise to bring her to orgasm by hand first, and then screw her afterwards, otherwise they tend to hump on your dicc, and you will often fall out of her. Feel free to cum inside her. You will enjoy it immensely, and it is impossible to get her pregnant on human semen.
Don't feel constrained to wait for her next season for sex. Between heats, she will still be receptive to sex, but you may find she is a bit tighter. You will probably have difficulty inserting your penis fully into her. Don't force it; you don't ever want to hurt her, betraying her trust in you. Dogs aren't built for missionary style. If you try that be prepared for frustration. It is hard from what I heard to enter them from that angle.