At every forward step and movement, at every going in and out, when we put on our clothes and shoes, when we bathe, when we sit at table, when we light the lamps, on couch, on seat, in all the ordinary actions of daily life, we trace upon the forehead the sign. If, for these and other such rules, you insist upon having positive Scripture injunction, you will find none.
St. Cyril of Jerusalem (313-386)
Let us, therefore, not be ashamed of the Cross of Christ; but though another hide it, do thou openly seal it upon thy forehead, that the devils may behold the royal sign and flee trembling far away. Make then this sign at eating and drinking, at sitting, at lying down, at rising up, at speaking, at walking: in a word, at every act.
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.
Pope Innocent III (1198–1216):
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...
So we can see that the earliest form was in fact crossing yourself on your forehead with your thumb. This later changed during the time of Eutyches in reaction to the Monophysite heresy. This changed occurred sometime between the 2nd/3rd Ecumenical Councils and the Fourth Ecumenical Council. This makes sense when you think about how we cross ourselves, with the pinky and ring fingers symbolizing the dual natures of Christ...
Also, what I found about the "Old Believer" practice, was that was the way Russians crossed themselves, and historically we can see that this was different than the "traditional" way. I don't think that made it wrong, and while the "reforms" were implemented very wrongly, the ancient way seems to be the three-fingered practice.