Establishing the link between The Witch and Hekate may take more than three invocations; that is going to be up to the individual. You will either feel it/know it, or you won't. One can, of course, proceed as if everything has worked as intended, even if one felt nothing. Fake it until you make it, as it were. Nothing dire is going to happen (most likely). The invocation can be repeated as often as desired. It can be performed for more than the three day cycle, but it should always be in multiples of 3 (3,6,9,12, etc) and should always culminate on the dark of the moon or on the feast day. Continue the offerings to Hekate, it never hurts to have a powerful, ancient diety on your side (or at least not pissed off at you). After performing the Invocation, The Witch becomes The Witch Enodia, a syncretism of The Witch and Hekate.
If you have already done the Initiation Ritual with The Saint and felt that it went off well, you don't need to do another one; but you may want to. If you want to do the Initiation of The Witch, it proceeds much the same way. Begin it after your Invocation of Hekate. I'm going to crib from Tommie here, with some minor alterations. "Iô" in the prayer below (pronounced 'eeeoh'), is from the Greek and means "Come" or "Come forth". You will notice in the last line of the prayer, "me" is replaced with "us", recognizing the eminence of The Witch Enodia, The Witch/Hekate and asserting Her authority as well as that of the magician and making the two indivisible.
"On the fourth day and for the next thirty nine days use the following prayer, changing it each day to include the name of the remaining thirty nine Servants in alphabetical order. Make an offering to both The Witch and the Servant on each day."
I call on you, The Witch Enodia, to come to me so I may know you.
I am (say your name), the master and ruler of this domain.
I call you here so that you can introduce me to the Servant: (name of the Servant)
Who is known for: (describe the Servant in your own words).
Iô , Enodia!
bring (name of the Servant) forth so that I may recognize him/her/it,
and in return he/she/it will recognise us as his/her/their friend and master.
Conclude the Initiation of The Witch with another three days honoring The Witch Enodia. You may perform the Invocation of Hekate, but it is my recommendation to simply make an offering of fire and water each day and use the Invocation of The Witch Enodia.
I am the Queen
of earth and sky and sea
lady of the crossroads:
and patron of the restless dead.
Call me Polyonymous
and through me,
authority over all.
To continue with this method, always invoke The Witch Enodia before before employing any of the Servants. You have elevated Her, and all answer to Her. Give Her the respect She is due and She will reward you for it.
I'm going to leave off here with a bit of homework for you, if you choose to accept. Re-imagine The Forty Day Ritual in this context. I'll start you off with a hint: it takes 42 days.
And for Extra Credit, check out the next section of PGM IV, 2785-2890. If you really want to go deep, add this to your workings.
Inimicus Dei Constituitur