I’m going to assume your disciplinary efforts are confined to ordinary skin in the area between the waist and the knees, not to any of the interesting body parts in or near the midline; that’s a topic for another day. (For an area of particular interest to me, see Most Intimate Punishment).
Most authorities recommend not mixing alcohol with BDSM play. There are multiple hazards in this combination; the top is less able to read the bottom, the bottom less aware if physical damage is being done. The risk of combining alcohol with BDSM play increases with inexperienced partners who do not know each other well and varies depending on the inherent risk of the activity. Just as an example, it's hard to imagine anything going seriously wrong when one person gives another an over-the-knee hand spanking. In contrast, some implements (canes and paddles, for instance) can cause real harm, and people can and do die from bondage or gags that go awry.
Please be careful.
If you’re using something thuddy like a paddle, don’t whack higher than about the midline of the bottom. Above that you will be striking over a thin layer of flesh that overlies bone (the sacrum and coccyx). It’s a lot less fun for the sub to feel those blows over bone, and there may be some chance of unintended injury.
If you’re using something light that can’t project force below the skin surface, such as the infamous Loopy Johnny, then you can whack wherever you want, although it’s considerate to restrict your efforts to areas that the sub finds erotic. Many people who love being spanked on the fleshy lower part of their bottoms find only misery in being struck on the bony upper part.
If you’re going to be punishing inside the ass crack, use something relatively light since the top half of the ass crack has bone all along the top half and the bottom half has important bodily machinery.
Although any implement can damage the skin, sometimes permanently, it's particularly easy to cause injury with paddles, since they carry such force. In addition to skin problems, paddles can injure the bones of the lowest part of the spine, in the upper half of the butt.
The most important technique here is to swing the paddle approximately in line with the upper half of the victim's body.
In this photo, the victim is nearly upright. It's hard to paddle someone properly like this as you have to swing the paddle in an upward arc so that it whacks the fleshy part of the butt and stays away from the bony regions that are higher. The heavier the paddle, and the harder you swing, the more important this is.
It's good to have some support. Leaning against a wall is not a good method, however; you should be leaning on something, not against it.
In addition, the barmaid in this photo has the paddle high over her shoulder. I hope she lowers it before she gives him his swat. See the next photo for an example of better technique.
Consider the photo below, which shows much better technique on the part of both spanker and spankee. The victim is well bent over, so it would be hard to hit too high. The happy lady is holding the paddle so she can make a level shot, rather than angling down from above (baseball players will recognize the importance of a level swing!).
In sorority paddling stories, it's traditional for the pledges to bend over, holding their ankles or touching the floor with their fingers. This keeps the punishment where it belongs. For victims who are not quite so supple, bend over something (a stool, as above, or a table) so that you can be at the right angle.