The difference is in the motivation. We collect merits if the action is motivated by bodhicitta.
How sure can we be that we are motivated by bodhicitta?
We can't be sure for we operate from a mind polluted with the 8 worldly concerns, self-cherishing and the three poisons.
So in order for us to create merit, we need to tap from a merit's field that is pure, that is free of the 8 worldly concerns, free of self-cherishing and free from the 3 poisons. That is a Buddha's qualities.
How do we do that?
We act within the realm of activity of a Buddha.
How?
Simple: we can do mantras, pujas, prostrations, butter-lamp offerings, etc... or simply follow the instructions from our Guru if we are fortunate to have one.
In acting within the realm of activity of a Buddha we tap from His pure motivation, and we create merit.
Then the merit will have to be "given away", dedicated, because as much as we are concern we will destroy it with only one moment of anger, with only one polluted thought.
Thus we can't keep it, we give it away!
So how can we enjoy the benefits of having created merits if we gave it away?
Simple, because we dedicated our merit, we will get it back, this is karma.
But then, with a polluted mind, how can our dedication itself not be faulty?
Indeed, it will likely be faulty or lack sincerity, so we dedicate "just as the Buddhas do"?
In this way, our action of dedicating is un-tainted.