Evocation is the act of calling or summoning a spirit, demon, god or other supernatural agent, in the Western mystery tradition. Comparable practices exist in many religions and magical traditions. Invocation is inviting the diety or spirit within you.
Evoking or invoking you need to know as much as possible whom you call.Never evoke something you know nothing of itis foolishness like inviting a stranger in your house.
In evocation-
For the Goddess, stand in the Goddess position
Feet at least a  foot or two a part, arms away from your ears upraised palms up
I evoke thee Great Lady Mother of all ,Gracious Goddess of love,life, death and rebirth come , I invite you to come join wiht me in this sacred  rite
Hail and Welcome Beloved Mother
This is just one way and a general evocation to the Goddess.
The God position ,
If you have seen  pics of the Pharoah or Osiris
Stand feet a foot  or so apart
arms wiht palms out flat toward you arms crossed over your breast / chest area.
Hail Horned One Lord of life and  light and love
Gracious Father ever Shining bright
Consort of our Blessed Lady come join with us in celebrating this sacred rite
Again there are many ways
To evoke Isis  there is the Lotus position, Morrighan esp Badb their is the bird or crow position staning on one leg with arms spread out as  a crow. raven
If you re search some Goddesses have spec positions you may evoke Them in
Hecate-esp on Samhain you use your staff pound ground three times howling
Hecate Goddess of the underworld I evoce Thee now to arise welcome back from the realm of the dead  from the underworld…..