What is an invocation
Invocation is a process od summoning entities "inside" ourselves. Evocation calls the entity so we can make our demand. Invocation calls the entity within so we can temporarily or permanently gain new abilities or boost older ones.
Sometimes during the invocation it happens the Magician becomes temporarily possessed by invoked entity and starts talking without conscious control. This memory is usually lost after the consciousness is regained, actually dreams and possessions are related so based on how well you remember your dreams, you will also remember your possessions. You can also use a recording device during your invocations to check later.
Simple form of invocation
A simple way how to attune yourself to the entity and slowly gain its abilities is using its name as a mantra, repeating it again and again through the day. Most usual names for this use are names of deities, e.g. YHWH, IAO, Eheieh Asher Eheieh, AGLA. Sometimes the mantra can also contain an action or intent, e.g. Om namah Shivayah.
My "Spirits, beings of The Light and of The Darkness, don't forget me, stand by my side and do my Will" is also an invocation mantra.
Ritualistic form of invocation
There is also a way of temporarily boost your abilities in a ritualistic approach.
You close your eyes and repeats entity's characteristics while trying to picture the entity in your mind, say as much as you can and the whole incantation repeat 3 times.
Then visualize the entity looking at you and repeat the whole incantation 3 times as if you were talking to him. So if e.g. your previous incantation started with "He is Posmiol, the spirit of Venus..." then now it will be "You are Posmiol, the spirit of Venus...". Your visualization can start changing on its own, don't panic, it means it started working.
For the last step imagine you are the entity, looking from its eyes at you. Once again repeat the incantation 3 times, replacing "you" with "I", so it will look like "I am Posmiol, the spirit of...". Picture yourself as best as you can, you mean literally feel like you are really the invoked entity. During this process the possession usually begins.
Downside of the ritualistic approach
For the ritualistic approach you should have at least a decent visualization skills and you also should have something to visualize. Many deities are formless, spaceless, timeless and therefore incompatible with this method.