1. Select the snipe function and click on the enemy.
Your character will aim at the enemy. Accuracy is irrelevant at this stage. This action costs APs.

2. Increase accuracy by using APs for aiming.
There are four buttons appearing in the control box, each of them uses up a certain amount of APs to increase accuracy. You can check the accuracy by moving the cursor over the targeted enemy after each button press.
Don't forget that firing costs APs too, so if you want to fire in the same turn, make sure to not use up all APs for aiming.

3. (optional) End turn.
The aiming is not disrupted by ending a turn, and in the next turn, you can use more APs to increase accuracy even further. You can eventually reach 100% chance to hit if you aim (use APs) for a few turns.

4. Fire.
Either click the fire button, or click on the enemy (the latter allows you to check the current accuracy). Firing costs APs.


Hope this made sense, though it's technically the same information as what's in the manual.