Calling the Quarters
The Quarters (also sometimes called Watchtowers) are the guardians of your circle. They are usually directional and/or elemental in nature. The exact wording on how to call the quarters is varied, and can be adapted to your needs. You might consider these Watchtowers to be the solid corner-posts of your circle.
It's going to be very difficult to find better information than this:
Plenty of information about Ritual Quarter Calls
But here's a couple more examples of how the Quarter Calls could look like:
Airiel Celticflower's grimoire
Quarter calls for the dragon tradition
Crystal Grove Web - calling the quarters
Four Quarters Coven - Quarter Calls
Witch's Cauldron - Quarter Calls
If I haven't mentioned it yet, YOUR SPELL CRAFTING IS SOMETHING THAT IS UNIQUE AND PERSONAL - the more you "do" of it yourself - writing the spells, calls, finding out correspondences if you decide to use those and so on, the better it's going to work for you.
I am not giving these links and this material for you to copy - I'm giving it as inspiration and learning material - do as they tell you only just as long as you can't make your own material.
And, believe me, a "bad" work of yours is better FOR YOU than what the best the world's best known witch and poet could ever produce...
If you are not familiar with any specific Gods or Goddesses, you can always just call on the Lord and/or Lady. Over time, you can research the various pantheons and learn about the different Deities out there. You may invoke certain Deities just get to know them better, or choose Deities that represent the energies you want for your ritual. You should always be respectful, and ask Deities to join you.
Francesca De Grandis' Wiccan and Faerie grimoire
Lady of the Earth; Invocations
Invocations from "The Skies of Avalon"
Brandy Williams papers; Invocation (has also her own experiences, so that's nice)
Michael P. Garofalo's collection
So, the homework this time is to write a quarter call and invocation of the God and/or Goddess of your choice.
If you go and check out the worksheet, you will notice, that this "higher power" that is being invoked or called here could as well be an ancestral spirit (or any spirit), an angel or basically what ever magical being/creature/power that you wish to assist you in your magic. (Yes, it can as well be a dragon...)