How believing works is that you reach the conclusion by a way of choosing the answer, resulting in an opinion. (as distinct from being forced to a conclusion by evidence, resulting in a fact). The painting is either beautiful or ugly, you reach the conclusion about it by expression of your emotions with free will.
If you choose the conclusion that the painting is ugly, then you are saying you have hate in your heart, which chose the words that it is ugly.
The meaning of this is, that the existence of the hate in your heart must be regarded as a matter of opinion, in order for it to be true that what is ugly or beautiful is a matter of opinion. So to say the painting is ugly, is to say you *believe* you have hate in your heart for the painting.
So you see believing is an opinion on what it is that makes a decision turn out the way it does. Who are you as being the owner of your decisions? That question is a matter of opinion, you can only reach a conclusion about it, by choosing it.
That is how subjectivity works, believing. You can practise that, in regards to other people. Look objectively in a matter of fact way to discover how they choose, and then you look subjectively, forming an opinion, about who the other is as being the owner of all those decisions.
If you have no religious training, you would want to not start with belief in God the holy spirit, but start with belief in the existence of the plain human spirit first.