Talk to your doctor about the soft stool. It may be a result of a medication you're taking, potentially serious condition, food intolerance or unbalanced diet. Seriously, it's something that needs to be checked out.
If you are healthy and have a bowel movement earlier in the day, there shouldn't be a need for an enema. There may be a few specks of fecal matter on the penetrating object, but that can be solved easily by using condoms (something you should use anyway to avoid STD's, UTI's for the giver, and keep toys/objects bacteria-free). Frequent enema use can lead to digestive and kidney problems and constipation.
Occasional enemas are fine. Use plain, warm water (ideally, body temperature). A quart or two should be sufficient for most people, and you may want to break that up into several smaller enemas. Doing several smaller enemas may help with the water coming out - if you're doing this a few hours before sex, you shouldn't need to completely evacuate your bowels and effectively rinsing out the first foot or two until the water comes out relatively clean should work fine. You're probably getting dirty water because you've been going too deep, but haven't done enough to completely clean everything out.
Be aware that the more water you use, the harder your kidneys will have to work and more you'll have to pee during/after.
Do NOT add soap unless instructed by a doctor. It kills the good bacteria that aids digestion and wards off infection and can greatly irritate the lining of your bowels. It's wholly unnecessary and can cause severe cramping.
You'll probably find laying on your left side with your right leg bent and pulled up a bit to be the most comfortable position. It also works best this way because it helps the water follow your anatomy, which is why medical practitioners give enemas and exams in this position.
Go relatively slow to allow your body to adjust and minimize any cramping. Hold the water until you feel you must expel it, then do so. Repeat until the water comes out fairly clean, and try to expel any water before you have sex. But if there's enough time between the enema and sex, your body should have time to absorb any extra water.
Again, the main thing is figuring out why you have loose stools. If you can fix that, you'll probably find you don't need an enema unless you haven't gone to the bathroom that day or are a little constipated and still want to have sex.