The philosophy of sex transmutation is best embodied in the last two sentences of your quoted text:

[Sexual energy] should be given an outlet through forms of expression which enrich the body, mind, and spirit of man. If not given this form of outlet, through transmutation, it will seek outlets through purely physical channels.

Also called Sublimation, the idea of transmuting an emotion or desire is not as mysterious as it might initially seem. If you are angry, and you redirect that anger energy into something like playing a sport, you are also exercising this practice.

So the idea is to redirect that energy, to channel it into a different life activity for the purpose of creation. In a marriage, this energy binds the two partners together in emotional intimacy (if it is used correctly). In a family, this energy is the seed by which children are created. Can you think of anything more fundamentally creative than that?

In the practice of an art form, this energy becomes the creative impulse by which mastery is achieved, and artworks that transcend ordinary experience are borne. Art which expresses the emotion of passion is universally understood by everyone.

When sex energy is dissipated through its usual outlet, in a casual way, it simply becomes the light of a burning flame, and then it goes out for awhile. There's nothing wrong with that; in fact, it's a great stress reliever. But if you want to achieve more with this energy, you must preserve it and redirect it. In so doing, it becomes more.

Thinking about sex the way N.H. discuses is like a computer that has a program running in the background.You don't really notice it, it's not bothering your other activities.and your wouldn't notice it unless you were trying to use all the resources of your computer. But if you could disable it from that start up your computer would run fast and more efficiently and if and when ;) you need it you just need to turn it on.

peace

How practical are you wanting? Would the creation of art be considered a practial application? On the one hand, art can be a physical literal form of expression so it isn't imaginary in a sense. On the flip side, there may not necessarily be a lot of usefulness to art and thus it may not be seen as practical from that perspective.

A guy could write daily poems or draw images if art is allowed as a practical application. Otherwise, one could see this as a motivating factor for someone in their job to do great work instead of merely acceptable work may be another kind of application here.

It is conscious control in the sense that one has to physical draw or write or whatever they want to do to utilize that energy. I don't see demonic possession being quite that common for another form to take on those energies if we don't control them properly.

A practical approach.
Transmuting the sexual energy into mental energy is easier than you've thought.
It is done by physical action. Precisely speaking by the postures of yoga. They are called asanas and redirect the flow of the sexual energy, channeling it from the sexual organs centre to the head. Yoga is an advanced system of spiritual development but it was greatly simplified by the Western world, maybe because there was a need in some point of the past for something "oriental" yet healthy. That's why most people links yoga with plain gymnastics.
There are 3 main asanas for that: Sirshasana, Sirvangasana and Paschimottanasana. Just 1-3 minutes of each every day is enough for the beginner. The first one is tough to do but I've mastered it after some time so it's not impossible. ;)
If you are interested here's the link, just scroll it down to the asana part.

click here