With each contact your perceive the experience: a pleasant, unpleasant or neutral feeling arises which you also perceive as either favorable, unfavorable and neutral. "Guarding the sense door" is you are aware of the sensation that arose and the perception that arose. Ideally you should be equanimous towards this experience, i.e., not attached or averse to the sensation and stimuli and also realise the evaluation you have given and feed the perception. If not (i.e. you react with craving, aversion or ignorance) then fabrications form: in which case you have be be aware of the fabrications also. These manifest as physical feeling in the body, having the characteristics of the 6 elements; and the mental component has the content 50 cetasikas (52 - 2 = 50 as feeling and perceptions are also cetasikas) -- see section on The Cetasikas in The Abhidhamma in Practice by N.K.G. Mendis
Also note the initial experience, which even if you are have been equanimous has a bodily aspect which you have the characteristics of the elements. The sensation is three-fold (are pleasant, unpleasant or neutral). Likewise the contact, as well as the subsequent reaction of craving if present, has an impact in all aspects of the 4 Foundations of Mindfulness -- for more details on this see Satipatthana related Suttas and literature.
The framework to guard your senses is outlined in the Cha Chakka Sutta though there are more comprehensive dispositions also in other Suttas which covers other aspects also e.g. Satipatthana Sutta Dhammanupassana section.
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Latent tendencies
LATENT TENDENCIES ARISING THROUGH THE EYE. Bhikshus, dependent on eye and forms, eye-consciousness arises.
When the three meet, there is contact. Dependent on contact, there is what is felt as pleasant, or as painful, or as neither pleasant nor painful.
When one is touched by a pleasant feeling, one delights in it, welcomes it, remains attached to it. Thus one’s latent tendency of lust (rāgânusaya) lies latent.
When one is touched by a painful feeling, one sorrows, grieves, laments, beats one’s breast and falls into confusion. Thus one’s latent tendency of aversion (paṭighânusaya) lies latent.
When one is touched by a feeling that is neither pleasant nor painful, one does not understand it as it really is, the arising, the passing away, the gratification, the danger, and the escape with regards to that feeling. Thus one’s latent tendency of ignorance (avijjā’nusaya) lies latent.
Bhikshus, that one could make an end of suffering here and now, without abandoning lust for pleasurable feelings, without removing aversion towards painful feelings, without uprooting ignorance towards feelings that are neither pleasant nor painful this is IMPOSSIBLE.
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Abandoning the latent tendencies
ABANDONING LATENT TENDENCIES ARISING THROUGH THE EYE.
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Bhikshus, that one could make an end of suffering here and now, having abandoned lust for pleasurable feelings, having removed aversion towards painful feelings, having uprooted ignorance towards feelings that are neither pleasant nor painful this is POSSIBLE.
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Also in many other suttas the following passage appears. The implication is do not crave and grasp the sensory experience.
Here, bhikshus, when a monk sees a form with the eye, he grasps neither its sign nor its detail.
So long he dwells unrestrained in that eye-faculty, evil, unwholesome states of covetousness and displeasure might overwhelm him, to that extent, he therefore keeps himself restrained.
He practises the restraint of it. He guards the restraint of the eye-faculty, he commits himself to the restraint of the eye-faculty.
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Nimitta and Anuvyañjana by Piya Tan