First, while I think shooting with both eyes open is better than shooting with one eye scrunched closed, it's not the be-all end-all factor in shooting. Plenty of successful shooters close one eye.
Second, slow static shooting at 15m and 25m is a lot different than reactive shooting at closer ranges. There are people who normally shoot with both eyes open but would then squint or close one eye when shooting at those distances.
But the technique for acquiring your sights is pretty much the same regardless. Look at the target, specifically focusing on the smallest part or point on the target that you can see. Then you raise the gun and as the gun interrupts your line of sight, you pick up the front sight and focus on it. You drive the front sight into the point on the target you want to shoot.
While other techniques (focus on the target, "soft" focus on the front sight, etc) can be used under some circumstances, they're not really well suited to 15m and 25m distances. The front sight is going to be the key to accuracy at that range, along with proper trigger manipulation.