Hi Petra,
Unfortunately I am overseas at the moment and the internet is running really slow so I cant do a search and paste some links for you to look at.
Perhaps you can type in "layering" here in Ausbonsai search or google and I am sure you will find a wealth of info pictures etc on how to go about it.
(The only caution being...be prepared as I am sure you will get a heap of "educational sites" come up after you type this description into google

)
There are a few different types of layering (air layering and ground layering being a couple, but within each of these types, there are a heap of different ways of going about it)
I only do air layers and the method I use is to cut a ring right through the cambium layer (about as wide as the trunk or branch is thick). You will find heaps of other alternatives - everyone has their own preference
I either place a plastic pot of soil or spagnum moss around the cut area.
Often I dont bother wrapping it in glad wrap, but that is because I am lazy. If you dont wrap with gladwrap then you need to water the pot as you normally would - perhaps even more often.
The safest and probably the best way is to gladwrap around the pot (sides top and bottom) giving you a better solution. If you wrap it tightly enough you dont need to water.
Incidentally, if you dont have a cheap plastic pot lying around, dont worry, you can simply wrap spagnum moss in gladwrap around the tree and it will work really well also)
The trick is to make sure you cut well past the cambium layer depth otherwise the tree simply heals over and your air layer is non existent
The end result is that a layer of roots develop on the top part of the cut and when you can see roots growing through the pot or spgnum moss, soil etc, then you simply cut beneath the pot and whammy - instant tree.
The advantages being that you can air layer at an interesting part of the tree and also get a nice nebari going as teh roots tend to be uniform around the cut.
Again, sorry I cant be more helpful at teh moment, hope I havent confused you
Steve