I cover evocation extensively in the sorcery course I teach, but for simplicity's sake, I'm going to give you all here the bare bones basics of evocation:
First off, there are 6 primary stages to evocation. Spiritual Authority, Staging, Conjuration, Manifestation, Congress, and Dismissal.
Stage 1: Spiritual Authority: this is basically a prerequisite to the evocation itself. During this stage, you call down a form of authority, whether it is from assuming a godform, invoking the vibrations of a name, or any other form. This ensures that the spirit will obey.
Stage 2: Staging: make the spirit feel at home. Set your ritual area with things attributed to that spirit, and make the appropriate offerings.
Stage 3: the Conjuration: this is the part where you call the spirit forth. The meat of the ritual, the conjuration is the part where you summon the spirit, and call it to manifestation.
Stage 4: the Manifestation: this stage has its own sub levels of manifestation, which we won't go into here. Suffice to say, this is the part where the spirit makes itself known to you.
Stage 5: Congress: here, you will commune with the spirit. Make your requests, ask your questions, have your visions, whatever the goal of your ritual is, this is where it occurs.
Stage 6: the Dismissal: some say don't banish after an evocation. I say, that's up to you. You may or may not want to banish, but you should always perform the Dismissal, as it informs the spirit that your official time is over, but they can remain behind to work with you if they wish.
These steps can be used with any system, all you need is a bit of flexibility and creativity.