For casting a circle can exist a lot of methods:This is mine:
I made a circle with stones.In it I put different magical object (candles,crystals etc) but before I put the objects in the circle I blessed it.Then I bash all evil spirits,there are different methods.For bashing I blessed the circle,then bashing with a broom.How I bashing with a broom?I push around the circle with the broom. If you want to call a deity or more you must have a statue,or a picture of even a candle.You put in a place (in your circle).You can say "(Deity name) I welcome you in my circle and thanks for all blessed given"I like to drink blessed water in honor of a deity,then say prayers,give offerings and do a spell.At end you must thanks deity for coming.
You can also invite the four elements in your circle..You can have a symbol of them or say "The four elements:water,fire,earth and air I welcome you in my circle".At end thanks to them.
When I cast a circle I like to meditate a lot,is very relaxing especially with a lot of energies around you.
At end when all is done,you must bash again with the broom the bad spirits.
A circle can be outside or inside.Don't matter very much.