Usually your soap nuts would come with instructions of some sort. Usually it'll be something like 3-4 nuts (or equivalent halves) per wash load (I remember also reading to use about the size of a small egg in the palm of your hand). You just place your nuts in the bag they provide (or a knoted up sock) and throw in the wash. Usually you can use the same nuts for 3-4 loads, depending how hot your water is and perhaps just how dirty your last load or two is. You do need at least warm water to release the saponin from the shell (that's what does the cleaning) and hot water works best. So if you're washing in cold you would have to soak the nuts for a couple minutes in hot water first and then put the water/nuts into the washer. Some also boil a few nuts in some water and use the concentrated liquid instead. You'll find lots of additional instructions either with the nuts you purchase or on any website that sells them.
No build-up, that's the beauty of them!
They absolutly do work well. Now, they don't have bleaching agents or UV brighteners so your whites won't come out gleaming like you might be used to with mainstream detergents, but they'll be clean. To get whiter whites they say you can add baking soda or an oxygen bleach. There's also not really any scent, so if you like scented laundry then you can add a couple drops of essential oils onto the bag for a pleasant smell.
A few of us have found that adding a tiny bit of washing soda helped the nuts work a smidge better but from what I've seen most people use just the nuts alone and they work great.