I think there are two ways to make the third practice you list more potent. That is:-
1.To intone the bija mantra mentally/silently,as supposed to chanting or whispering, Ommmm( somewhere i have heard it said as sounding more like Ong when it is listened for) I would’nt advise doing this for more than a couple of times- up to 2 or 3 is likely sufficient to activate enough vibrational energies for an average state of purification.
2.To locate the pulsation right where Ajna proper/Third-Eye central is located which is more or less above and behind the nasal septum,top of the wind-pipe then,throat etc in the locale of the pituitary and pineal glands( it may be called ‘cave of brahmin’ or some such thing in some sources). The brhamadaya as it is called, located between the eye-brows, is i have read, more where ajna chakra is felt.It is an easier exercise to put, place our awareness there.
Also it worth bearing in mind that all the practices of a tantric sadhana are not practiced of course in isolation- that all the chakras need purifying.This takes alot of patience and cultivating sensitivity to the flow of pranic currents throughout the subtle body.The Kundalini Tantra method outlined in that book is to start first with ajna( the control centre) and muladhara, then acending upwards in sequence.
For safety reasons the effects are negotiated over a times-scale of they say a month for each chakra(so 8 months total for purificatory stage) before any kriya yoga practices take place( and then they recommend the guidance of an experienced teacher or guru)
Of course this is all for safety reasons.
I am working on a tantric sadhana myself based on the book ‘Kundalini Tantra’ by Swami Satyananda Saraswati.It is also worth getting to grip with the shatkarmas-methods of purifying body and mind.I do have experience with a complete or full scope meditation-based yoga system called AYP( see their website aypsite.org for more details) so i would hardly say i am out of my depth.
The only drawback ,if there is one, is that it can take a certain amount of time,patience and discipline to purify & awaken your kundalini sufficently.But things worthwhile in life often do not come without some effort.I would say however the investment in the time and energy is well worth it.Opening the doors to one’s full potential and kickstarting one’s personal evolution, i could’nt think of a better project to engage in.
A Tantric sadhana is basically a sadhana that tries to incorporates and integrates within a broad spectrum practice approach most of the main yogas.Meditation ,sometimes on the chakras/psycho-energetic centres of Consciousness,within the subtle body ( laya yoga), is a main key to that endeavour.
As i say, it can take some time and effort,patience and discipline( i.e structured & self-paced ,self-directed practice) though, but it is well worth it.
(Note:-Also to add here- the explanation Benthino offers is a reasonable one- as the chakras are often likely felt or intuited via ajna ,ajna being the control centre for the others.It is probably best the crown ,for that matters,opens by itself naturally ; that would seem the safest idealist way, to me.Might not be a huge deal but probably best getting some opening in ajna first before proceeding to the others and working your way up.You learn to feel the currenst and pranic movements in your psychic subtle anatomy as you practice consistently and move along-gain experience,progress etc baby steps ideally.You are basically gently prizing open your own nervous system and it’s internal dynamics( through ‘active surrender’) through a gentle but gradual and systematic [I]nudging[/I] process)