Well first thing you need to know about that is that every guy is different. Some guys need more preparation than others. Second, given I'm guessing it's both your first time? So you should know it's going to hurt a bit for whoever receives until they adjust and things get going. And yes, they'll be sore afterwards. It's normal and natural, and things will get better the more experienced you two get.

But, there's ways to minimize the discomfort. The grocery store has lube. Thaaaat's... in the isle where you can find the razors and other grooming aids. Use that to slick up the penis of whoever's going to be penetrating, and then get the rest on your fingers. That's got to be worked inside whoever's receiving. [He's going to demonstrate now a scissoring motion with his fingers.] Use one finger to start and steadily work your way up to three. Stretch the entrance with the movement I'm demonstrating now; it will make things easier when the act finally happens. Generally, the more lube used the better.

Also very important. The guy receiving needs to relax. Tensing up in the middle of this makes it worse. So the other guy needs to be sensitive to how their partner is taking it, and ease up if needed. Communication is pretty key here. It makes things a lot easier.