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.
Peter of Damascus (12th Century) gave the following instruction: