If you feel quite comfortable and enjoy your basic Kriya Yoga practice ( you should be able to to do now at least 108 breaths with with ease and without any strain) you can boost up the exercise significantly with adding Mantras (Words of Power) to your Kriya breathing.
I would suggest as a starter to choose one of the following Mantras :
Om Namah Shivaya
Om Mani Peme Hung
La ilaha ´il Allah
These are widely used Mantras of Hindu -, Buddhist- and Sufi – Yogis. They are highly charged and you don´t need a special initiation or permission to use them. Generations of practioners have created through billions and billions of repetitions of these mantras a very powerful energy – pool. Plug in and use it to support your spiritual development !
Choose one of the above Mantras to which your inner Being feels the greatest resonance. Of course, if you are initiated into a personal mantra and this mantra is in relation to the lenght of your natural breathing - cycles not to long, you can use this one.
The mantra can change from time to time , especially when you feel that the mantra is dead and don´t resonate anymore with your inner being. Always choose a living mantra for your practice! But as a beginner ( and also experienced practioners should learn to cultivate a fresh beginners mind ) try to stick to one mantra for a longer period of time, because it takes time to build up the divine connection and personal energy pool. So don´t hop from one mantra to another on a daily basis or during an exercise. Each of above mentioned Mantras can lead you to Liberation , if you are practicing them with a sincere Intent and (joyful) discipline on a regular daily basis.
So choose your mantra and repeat it silently (internally, without use of your voice-chords) with each Kriya – inhalation and exhalation. Synchronize one repetion of your Mantra upward your spine with your inhale - and one repetion with your exhale downward your spine. Synchronize the sound of Mantra with the natural lenght of your Kriya breath , not the breath to the repetion of the mantra !
If you have problems with the silent use of mantra , you can sing or chant them (without kriya breathing ) alone or with likeminded friends in a private atmosphere ( i.e don´t disturb anybody or try to show off in public , how “spiritual” you are). In my personal practice I like to chant , when driving my car and there is no heavy traffic. Although all my friends and relatives agree , that I am a horrible singer, - I don´t care - and have always tapes, CD´s , mp 3 with my favorite mantra tunes to sing along in my kitchen or car. I have added to this post exampels , how to chant and pronunciate the above mentioned Mantras.
At last never forget the three pillars of spiritual practice :
Kiss ( Keep it simple , stupid) .
and kissing your Beloved is always the best practice