Paul Huson, author of Mastering Witchcraft, probably has the best guidelines on casting spells, in his section on the "Witch's Pyramid". As my copy isn't readily available at the moment, I'll forego quoting it, and simply refer you to it.

It's been in print nearly continuously since it was first published, and no less an authority that "Catholic Weekly" considered it "a genuine vade mecum", or "do it yourself" guide.

I will offer one simple spell for you here, written many years ago by an Elder in our Tradition, Merlin:

Oh man of magick weak
Pray hold thy tongue and do not speak
Of things beyond they feeble ken...
Oh rodent among rodents
And mouse among men.