As often happens, there is a huge difference between what is sexy and what can actually be done. Torture with electricity is a very dangerous practice, and can easily lead to death if not handled carefully and under medical control (I know that it sound absurd, but during interrogations in the Chilean and Argentinian times a doctor was normally in the room.) Electric current behaves differently in each human body, and there are intrinsical differences in health, age, conditions of the circulatory system, prsence of fat, etc. A relatively small amount of current through the heart, for example, can be fatal for many individuals. Don't try this at home.
The general advice within the bdsm community is to always ensure that all electrodes are below the waist to prevent a current path through the heart. Unfortunately a lot of bdsm porn movies get this wrong, and there are connections to the breasts which is much more dangerous. To do this (more) safely, you need two connections to each electrode so that the current has only a short distance to travel between the electrodes.
Nipple electrodes in particular, need very careful thought to make them low-risk (I'm not going to use the word "safe" here, because electro sex is never 100% safe). In general, you need both sides of the circuit connected on each side, and make sure that one side is totally isolated from the other.
This shows the correct way (apart from the needles piercing the flesh which is a very bad idea for e-stim as blood conducts electricity far better than skin does, so you run the risk of unpredictable current paths through the inside of the body)
Note the red and black wires both go to the one set of nipple electrodes. For this to be low-risk, the other wires going to the other nipple need 100% isolation from the first one (common earth connections are highly dangerous in this sort of thing - most commercial e-stim machines use isolation transformers to make sure that there can never be a current path between the output channels which, in this application, could form a current path across the chest)
Do NOT do it this way;
In this example, we only have a single channel with the opposing polarities connected to different breasts. If you do it this way, then sooner or later, you're going to kill somebody
This is about as dangerous a configuration that you can do in e-stim
Particularly dangerous are those nipple clamps you see in porn movies where they are basically a pair of wooden sticks, one of which has copper wire wound around it, There are no different to directly clamping the nipples as in the second picture above. Fortunately, these days, most of these clamps have wire wrapped on both sticks and each side wired seperately. This is the same as in the first picture, where each nipple is stimulated seperately and isolated from the other.
If you want yo do this stuff I would advise that you do your research, and buy the proper equipment. If you're going to go the DIY route to save a lot of money, then make sure you know exactly what you're doing - and beware of unexpected current paths. In the first example, the current can only flow through the nipple in question and nowhere else, as electricity will always take the shortest path. However, if one of the wires should break or become disconnected, then there is the potential for the active side to indirectly couple to the other breast, resulting in an unexpected current path. Commercial e-stim equipment lowers this risk due to the both channels being isolated due to the transformers, but DIY equipment may not have this level of safety isolation (frankly if it doesn't, then don't use it) and this also applies to some of those cheap wun-hung-lo knockoff products you see on ebay.
Be careful, read up on e-stim and don't take chances.
Also if you are ever going to do this sort of thing, at least learn about electrical safety and make sure you thoroughly check your equipment before you use it - your life (and your sub's) may depend on it.
(Sorry to sound patronising but safety is always the first rule in all aspects of bdsm - as always, if you're not sure about something, then don't do it)