Well, if you catch your cold during the peak cold season in January and February, its too late to go gather your own elderflowers and you have two options: you can buy loose dried herbs from a good supplier, such as Neal's Yard or Napiers (who do internet and mail order, see 'Interesting Links') or you can buy readymade organic elderflower tea bags.
To make the tea, use 1 heaped teaspoon of dried flowers or 1-2 teabags per cup. Leave your tea to brew for 5- 10 minutes, and drink it hot if you are taking it for a cold.
Note: The important thing about making a medicinal tea is to cover it while it brews. If you don't want to use a teapot, then placing a saucer on top of your mug is just as good .. you can get those little metal strainer-balls to place the herbs in. I use a lovely pottery cup which has its own lid and a little tray to hold the herbs. Its worth it if you make a lot of herbal tea.
As a regular drink, you can take 3-4 cups a day although the book I use suggests you can have 6-8 cups a day (for a FEW days only!) when you have a cold or flu.
If you are interested in collecting your own elderflowers, then the optimum time to do this is in May or June, just before they are in full blossom. Collect whole flowerheads and spread out to dry on trays in a warm place out of direct sunlight. When dry, the flowers can be easily rubbed from their stems. Store the dried elderflowers in airtight glass jars in a dark place. Dried herbs should be discarded after a year and replaced.
'Natural Taste (Herbal Teas: A guide for home use)' Andrew Chevallier MNIMH (1994)
'Healing Plants' L. Opletal and J. Volak (1999)