Given the size differences between Mobians and humans, I’d say slowly and gently. Play with her first and start penetration with two fingers inside and carefully spread them apart to loosen her up, make sure she’s good and wet and put the head of your penis in first. Don’t thrust, go slow and with each push putting it in a little deeper until she can take all of you. When she’s worked up enough and comfortable you’re both good to go nuts with the fucking. Don’t worry, Sally (probably) can’t get pregnant from your human seed so have fun!