To prepare a tea, mor exactly: a herbal tea, make water boil and put in the herbs, quantity according to your taste or, if it is for medicinal purposes, according to needs. Teas are taken both for their taste and for their properties.
If they're available, the best option is to take fresh plants. They can be dried for seasons when fresh plants are not available, like on Ibiza the stinging nettles, which (as therophytes) pass the summer only as seeds, and so in order to have nettles in summer, they can be collected in spring and hang up in a shady place with fresh air.
With some plants with essential oils like the mints it is important to turn off the fire before putting in the herbs.
With plants with a strong taste like rosemary, if the tea is left for some hours, it is adviseable to take the plants out after twenty minutes (or more or less, according to one's personal taste). Other plants like nettles, mints and sowthistles can be left in until the tea is finished.
Sometimes it is asked if twigs with flowers dan be used (f. ex. rosemary twigs). Of course they can, the flowers do no harm. In some cases like with the poppy, it is just the petals that are used. Also fruits like the carob can be used for teas.
The herbal tea in the ABC of Medicinal Plants (in Spanish)