How to dry Eucalyptus leaves (to later on make herbal tea from)?
You can dry herbs, chillies etc. in an oven, if your oven goes low enough. A dough proving setting is good though you can go just a little warmer. If the door makes a tight seal the moisture can't get out, so it's worth opening the door just a little once it's up to temperature. 

If you find you're making a habit of drying food, a food dehydrator is a good idea. This is especially true for strong smelling foods as you can put the dehydrator somewhere well ventilated - if you dry chillies in the oven they will make your eyes water every time you go in the kitchen, and it wouldn't surprise me if eucalyptus was similar.