Having the lily flowers on wire stems makes them very easy to arrange. The pink lilies shown here are a variety which grows with a single flower on each stem and always turns up to face the sun. When you arrange them, use your tweezers to bend the flower away from the stem so that it gently faces up or slightly to one side.
Some lilies (turks head lilies for example) always face down, and are grouped on a stem. To make turks head lilies, curve your petals very strongly when you shape them. When you have formed the flowers, cut the stem about 1/4 inch long beneath the flower, and glue several flower to a main stalk in the pattern in which your lilies generally grow. (Usually a slightly spiralled arrangement like the leaves). In most cases your main stalk will be a single stem with a flower on the top of it. You can cover the glue join where the base of the flower meets the main stem with a bit of acrylic paint once the glue has dried. For turk's head lilies (tiger lilies etc.) which face down, shape the flower on the main stem you made it on before you cut it free. Then you can glue it to your main stalk already facing down in the correct direction. Turks head lilies also have longer, very pointed petals, so you may need to trim your petals to make particular varieties of liles.
If you are making Easter lilies or Madonna lilies, you may want to make some flower buds for the top of your stems. The buds can be made by dipping the tip of the wire in glue to build up a bud shape (it will require one or two dips, allowing the glue to dry between dips.) the buds can then be painted white, and given thin green lines to mark where the petals will separate.