For starters, think carefully whether you really want to communicate with these mysterious beings. Remember everything you know about them. Who are the elves? In England they were called "little people" in Ireland – "seeds", in Scandinavia "Alf" in Germany – "dwarfs". This immortal creature, completely alien to people. They are not good and not evil, to others they are indifferent and only in the case if you have an interest. And the consequences of their aid is unpredictable. Remember the legendary Scottish bard Thomas the Rhymer (an ancestor of another great poet Mikhail Lermontov), who, after spending a year as the guest of the elves, returned to their world very old.
So, you've decided to enlist the help of the elves. But just like elf did not cause. They are usually hidden from the eyes of mortals and are only to those who will be of interest to them. So try to interest their potential interlocutors. It is best for this purpose learn the prosody and music of the Celtic bards. This, of course, will take time.
Now is the crucial moment – call the elf. Wait for one of the two most suitable days of the year. These days 1 may, the Holy day of the Celtic holiday "Samhain", and 1 November – day of the feast "Beltain". In these days elves, as well as other spirits and magical creatures, more likely to communicate with people. Best of all, of course, to meet this day in Ireland, England, Wales or in the French province of Brittany – there are still traces of Celtic magic. Pre-select remote from the cities of the place. It should be a hilly plain, preferably with the remnants of ancient pagan buildings. With the coming of darkness hide not to frighten off guests ahead of time, and wait.
At midnight, if you're lucky, you'll hear beautiful singing, and then see the procession of creatures of unearthly beauty. Go to them with a bow and call with your request. Here it would be best if you apply the learned skills of versification. Now everything depends on you.