(from Texts common to Early Buddhist schools)
First of all, get this visual-aid handy, all along - for it should definitely help.
A "sense experience" is when the external sphere of senses (bāhirani āyatanāni) reach the internal spheres of senses (ajjhattikānī āyatanāni). By the way, these "spheres of senses" - or "sense bases", are better translated through "fields of sensory experiences".
This happens when, for instance, the appearance of a tree, reaches the eye, (or more precisely the "field or ground of experience" that is the eye - viz. the sight as "experience").
A genuine Buddhist should not get hold of that sight, because, said Buddha, unwholesome states of covetousness or displeasure might invade him.
But how & why does one get a hold on a sight to that extent? What is the process involved?
In Buddhism, a "sense experience" involves primarly these āyatanāni (āyatanāni = plural of āyatana), and their indriyani (plural for indriya), which are the faculties attached to these ayatanani. We will call them indriya and ayatana, for simplification purpose, from now on.
इन्द्रिय indriya is basically a power. It is the faculty attached to an ayatana. This means that, without that power, the ayatana (the field of experience) is inactive, so to speak. You could think of indriya as the seed that goes on the field (ayatana) to produce a tree (sense experience).
Or another metaphor would be the muscle (ayatana) of this poor frog that you dissected in your natural science class, and on which you applied an electrical current (indriya) to produce a reflex (sense-experience).
Without indriya (power/faculty,) there is no active ayatana, and therefore no sense-experience.
There is what is called a descent (avakkanti) of the indriya (power) [see here] , in the empty, neutral & inert ability that is the internal āyatana. (see in this extract in SN 35.238, how these internal ayatana (called here: internal sense bases,) are defined as void, hollow, empty (and then "attacked" (SN 35.238) by agreeable and disagreeable forms [like the tree of our example - ugly or beautiful - creating a feeling of repulsion or attraction]).
Forms, feelings (khandhas in general), but also the internal ayatanas, that are not "ours" says the Buddha. (SN 22.33 - SN 35.138).
Note: In echt-Buddhism, sense-experience is always a bad occurrence per se; for the mere reason that if it is good, it might trigger some covetousness - and if it is bad, some enmity. A beautiful car, for instance, will certainly create a feeling of envy and jealousy; while an ugly car in your posh neighborhood, might create a feeling of disgust and enmity.
Buddha speaks a lot about restraining the indriya. It is called "keeping guard over the doors of the powers (faculties) > Indriyesu guttadvārā," in the early Texts (Suttas).
In AN 6.55, Buddha recognizes that these indriya, these powers, these faculties, cannot be totally eradicated - but however brought to their plain, flat, regular, viz. normal level (see the definition of सम sama in the Monier-Williams Sanskrit dictionary for pre-Buddhist texts).
How do this indriya (e.g. the power, the faculty of sight,) descend (avakkanti) in the eye (more precisely, in the field of experience, that is the ayatana of "sight"), is expressed in SN 22.47.
And how to restrain (guard the door,) of this indriya, is explained by the following:
The less you identify yourself with the "self" - that is to say to identify with it (viz. the external - in our example the tree, and its external ayatana = form (rūpa) [see visual aid] - namely to make the external "yours," and appropriate it as "this is me", or "I am this" - then the less "infinite" is (the power of) the indriya.
Let's take another example.
Let's say that you are looking at a Picasso. You have the tendency to think that the sense-experience, and the feeling attached to it, is "yours". But, in fact, it is Picasso's feeling that you are experiencing. All that is asked from you, is to attach one of the three basic natures of feeling, that exist in Buddhism. Namely "pleasant," "unpleasant," and "neither, nor". As Buddha said once, our body is just "made to be felt" (SN 12.37).
Identifying oneself with the external is a wrong view.
Identifying oneself with the internal is also a wrong view.
Both are stages to be aware of, and to get rid of.
But that is another story.
Just know that getting rid of the senses ("external",) takes you to the "internal" - viz. to the *right* knowledge and the right use of the basic forms (earth, fire, water, & air), that is to say, to protect them (see the simile of the city - SN 35.245) .
Then, once you do not clung to these "form" (rūpa,) [first the external forms - then the internal forms,] you are able to get to the knowledge of the formless. So on and so forth. This is the process of liberation in Buddhism.
So, first get rid of the influence of the senses (through the restraint of the indriyani) - then know the internal forms (and don't get caught into the outcome) - then know the formless (and don't get caught into the outcome), etc. etc. Until you free yourself thoroughly.
This is a sense-experience, as per Buddha's words in the Suttas (with parallels).