Start with these basics -
I've always used hairpins, but you can use straight pins too. I like the hairpins best because you can just pull a candy off the tree and the pin stays in place. You don't have to worry about discarding a straight pin.
Start with the bottom layer
The closer you get to the top, you will need to angle the pins into the styrofoam.
Cut pieces of thin green ribbon and pull them into a knot (not too tight)
which goes right into the tree.
Put as many or as few ribbons into the tree as you like. I loop ribbon into another hairpin and stick it into the top of the tree.