Using delta time messes up movement

Using delta time with my speed variables actually don’t make it so nice, after all. The movement is smooth, but still the speed is different on all computers.

I want to have some kind of a constant speed, but also a smooth movement, on all computers. My girlfriend has an older laptop and when she runs my game Sokobanski, the character really moves slowly, while on my computer it’s like the normal speed. And when my brother runs my game on his new powerful PC, the character there moves too quickly.

So, my question is, how do I create a smooth constant movement? How to make the movement same on all computers?