Freyja as Sorceress
Freyja is described as the greatest sorceress in the nine worlds. What that means exactly is more complicated than most would have you beleive. Descriptions of her powers are almost non-existent and the powers of most sorceresses (and the rare sorceror) are vague, fantastic or both.
Magic to the Ancient Germans is not terribly different from most shamanistic cultures and include prophecies, shape changing, potions and spells. The difference is that while, communing with spirits. gods and the dead are included in the magics of other cultures (and thereby only practiced by the initiated), the Germanic peoples treated those communications as no more'magical' than talking to the person seated next to you.
Most research on Freyja-inspired magic focuses on a kind called Seið. There are plenty of descriptions of magic that do not fall under the category of Seið - but many hopeful modern sorceresses use the term to refer to all magic that doesn't involve runes. This is because the lore specifically states that magic involving runes was called Gald.
Note: The final-R after Seiðr and Galdr are not required (actually redunant), though modern America tradition inexplicably removes it from Seiðr to be Seið but leaves it after Gald to remain Galdr. Here, the final-R will be omitted from both.
What is Seið?
Seið (rhymes with 'bathe') was most likely a form of trance-induced prophecizing that is described in two separate sagas. Surprisingly, both references are almost word-for-word and suggests that one was copied to create the other.
Seið means 'seethe' or 'rolling boil' and is surprisingly never described in the lore. Certainly trance-induced magical practices are mentioned - but not described. This shouldn't deter the modern student, but it should be accepted that all practices are reconstructed. Another term for Trance-work was Spá (rhymes with 'cow') or Spæ (rhymes with 'spy') which means simply "spy" as in "see" or "seek". Spácraft is descriptive of the practice while Seidh is descriptive of the technique.
The purpose of Seið in the described instances was to beget prophecies. In several cases, great spirits are summoned to speak prophecy - thus is the story of Voluspa and other greats lays. Other purposes or uses for trance-magic are supposed from weak lore references or gleaned from other cultures.
In brief, the practice of Seið involves the seeress called a Seiðkona ("Seethe-Woman") or Spámaður ('Spy-Man') induces herself into a trance. Another word for the seer was Völva (rhymes with 'velvet'). In some current shamanist cultures, questions are asked before the treance begins and in others the questions are asked during the trance. The Seidhkona then speaks about her visions in the trance that are, hopefully, related to the question that was asked. The specifics of how the trance was induced are vague. In one case (copied into two sagas) an assistant sings a song to induce the trance. In The Flyting of Loki (Larrington's translation), Odin is accused of practicing Seið and beating a drum 'as a witch'. It is not described whether hallucinagens were ever used, but botanical research shows that several were available.
There are two great references for those who wish to learn seidh for themselves. The first is the book Seidways by Jan Fries (1997). This is a wonderful book on Seid and Trance work because it goes beyond the purely spiritual and goes into some of the science behind trances, including brain-waves, etc. There are other books, but this is the only one I recommend.
The second major resource is http://www.seidh.org otherwise known as:
Diana L. Paxson's Site
for High Seat Seið
and the Core Oracular Method
Diana Paxson is a well known Heathen author of both fiction and non-fiction books. She is a member of Hrafnar kindred which founded a school of siedh that has come to be known as the Hrafnar Method. The Hrafnar method involves a song written by Hrafnar members to be sung to induce the seeresses trance. The seeress describes the vision of her journey to the best of her ability. Thence, querants are allowed to pose questions or needs to the prophetess and she then describes the vision that appear to her. Sometimes, those visions involve interaction with spirits or even gods. Some visions are cryptic and some are blunt. Each seeress seems to have her own style of vision, with varying degrees of description between them.
One should experience Seidh as an observer and then querant before tackling the techniques oneself. There are both better and poorer oracles out there, and the line between 'divinely inspired wisdom' and 'hokey cliche advice' is often blurred.
This practice is rather extreme to the non-Heathen community, and that it has been mistaken as both epileptic episodes and demonic possession. Practice should be limited to group experiments in a supportive environment.
WARNING!!! - Information gleaned by oracles are filtered through that person's personal knowledge and experience. It has been observed that the less knowledgeable the seeress in Asatru lore, the more conflicting and confusing the pronouncements are. To be a good seer, one MUST be versed in the lore well enough to know when a vision is Heathen and when it is coloured by non-Heathen symbolism. Some Heathen women have turned away from Asatru when their visions conflicted with the facts they learned in the lore. They made the mistake of thinking that their mismanaged 'filter' is providing them with 'truth' more correct than the lore, when the only 'truth' is that they prefer their filter to fact.
Freyja's Other Sorcery-Skills
Freyja owns a cloak of Falcon feathers, with which she may fly through the nine worlds (the only other god with this power is Odin upon Sleipnir). Whether it turns her into a falcon for flight or it turns into wings or something else entirely is not described.
Freyja is able to change the shape of other into beast - such as her lover Ottar into a boar. In the sagas, several sorceresses appear that change their own shape into beasts (e.g., seal, dragon, etc.). Both Odin and Loki possess this ability too. Loki is an ettin, and may be an innate ability. Odin is said to have learned some magic from Freyja, and this might be some of that knowledge - or it may not.
Freyja is never described as performing any trance-work herself. Instead, she summons other seeresses to prophesize for her. Odin performs the same trick several times, and this again may be knowledge learned from Her.
There are a few surviving 'spells' that invoke Freyja. Such spells won't be discussed here (yet). One should look up the Merseburg Charms and the Galdrabok for examples. There are also spells mentioned in poetry and sagas that are certainly not rune spells, and so might fall under Her auspices.
Some herbalists feel that their practice harkens back to ancient wise women who've been mistreated and exiled by the general scientific community. While that may have some merit, the scientific method of testing a drugs effectiveness before administering it has saved enough millions of lives over the centuries to warrant its respect. Personally, I'll take Dr. Scholl's wart remover over swinging a dead cat any day of week. Still, before medicine was a science, Freyja may have been worshipped as a healer of sorts though there is no lore recounting it. The demi-goddess Eir was the healer, yet if she used any form of magic, Freyja surely was capable of performing the same.
Despite the references to magic use, there are almost no detailed instructions on precisely how to perform any of them. Among the exceptions is the Galdrabok (translated by Dr. Stephen Flowers, aka Edred Thorsson), a collection of Icelandic spells and charms explaining precisely how to warp the world by one's personal will.
In the modern world, there are countless avenues for an esotericist to learn and experiment with arcane arts. To qualify that, however, it should be pointed out that among those practitioners, there is a noticable lack of millionaires, politicians, or famous persons of any ilk. Consider this before investing too much into esoteric arts.
Freyja provides the foundation for us all to earn our way. All we need to do is knowledgably apply our skills to that and success in any task will follow.