How specific should we be when confessing sins of impurity?
The basic norm for confessing grave sins is that we should be sorry for all of our grave sins without exception. For this reason the Church asks us to confess grave or mortal sins "by number and kind." This just means "what and how many times" you did what you did. The real immediate matter of the sacrament of penance is our contrition for sin, not our sins themselves. If our contrition is sincere and universal, that is the most necessary thing. Then confessing guarantees our sincerity and sorrow and so makes us more fit to receive absolution. So with this in mind, I answer your question about purity specifically:
First, you should say whether you are married or unmarried; and if you have taken a vow or promise of chastity or celibacy, you should mention that.
Second, you should say what it is you did and with whom, for example, intercourse with a married person, or with an unmarried person. If it was not natural intercourse, then you need to mention that, and whether the unnatural act was with your spouse, or with a person of your own sex, or by yourself alone.
Third, you should say how many times you sinned in this way.
You should also mention any circumstance that changes the sin in any serious way, for example relations with a prostitute, or with a consecrated person, or by force, or in public. These circumstances add other sins to the sin against chastity, like sacrilege or scandal or injustice.
You need not describe in any way how you did what you did, only the kind of sin it was, and how many times. For example, you do not need to say how an unnatural act was performed with yourself or another, but only that it was not natural intercourse. With the number, you do not need to torture yourself as long as you are sorry for every time you committed the sin and are giving the priest a fair sense of how frequent a problem this sin is.
All of the above applies to confessing deliberate impure thoughts—that is, you just confess how many times and about what you deliberately entertained impure thoughts. You do not need to confess the impure thoughts that immediately preceded or accompanied your impure actions, because the thoughts are then included in the actions. Viewing pornography is on the one hand a case of impure thoughts, but it is also a case of injustice, as you are contributing to people making a living as slaves to others. They are not free to chose a relationship with you, and you are using them.