This is how to bless someone with your hand and make the sign of the cross over them. Hold three fingers, as equals, together, to represent the Trinity: God the Father, God the Son, and God the Holy Ghost. These are not three gods, but one God in Trinity. The names are separate, but the divinity one. The Father was never incarnate; the Son incarnate, but not created; the Holy Ghost neither incarnate nor created, but issued from the Godhead: three in a single divinity. Divinity is one force and has one honor. They receive on obeisance from all creation, both angels and people. Thus the decree for these three fingers. You should hold the other two fingers slightly bent, not completely straight. This is because these represent the dual nature of Christ, divine and human. God in His divinity, and human in His incarnation, yet perfect in both. The upper finger represents divinity, and the lower humanity; this way salvation goes from the higher finger to the lower. So is the bending of the fingers interpreted, for the worship of Heaven comes down for our salvation. This is how you must cross yourselves and give a blessing, as the holy fathers have commanded.
The sign of the cross is made with three fingers, because the signing is done together with the invocation of the Trinity. ... This is how it is done: from above to below, and from the right to the left, because Christ descended from the heavens to the earth, and from the Jews (right) He passed to the Gentiles (left). Others, however, make the sign of the cross from the left to the right, because from misery (left) we must cross over to glory (right), just as Christ crossed over from death to life, and from Hades to Paradise. [Some priests] do it this way so that they and the people will be signing themselves in the same way. You can easily verify this — picture the priest facing the people for the blessing — when we make the sign of the cross over the people, it is from left to right...
I suppose it's really just a matter of custom, rather than an established rubric or a Law of the Church, but I think I also remember seeing it in the St. Joseph's Baltimore Catechism (the one for elementary-school-age children) as using three fingers as well. I may be mistaken.
Either way, two or three fingers, left/right or right/left, the point, as was made by St. Bernadette, is that the Sign should be made REVERENTLY, GRACEFULLY and with the INTENT of marking oneself with the Holy Cross of Our Lord.
The Sign should not be made carelessly, nor secretively. If one is confident enough in their beliefs to MAKE the Sign upon themselves, one should be confident enough to make it WELL.
St. Bernadette of Lourdes, pray for us.
Immaculate Heart of Mary, triumph soon.
Sacred Heart of Jesus, have mercy on us.