1. Chant aloud as it will be easier to continue chanting if u r chanting aloud.
2. Fit the mantra to a favorite tune or nursery rhyme. Sing the mantra aloud in that tune. This will be enjoyable, so you will be able to do it longer.
3. Keep reminders of the mantra all over your house, place of work, car etc...eg on the laptop screen, on the mirror, in ur kitchen, on the fridge, next to ur clock etc. If you dont want to write the entire mantra, then u could make it discreet by writing the alphabet 'k' maybe, which could represent the mantra. Every time u see a reminder, chant the mantra a few times, so over the course of the day, you will complete 20 mins of chanting. (Of course its best done 20 mins in one go, but if that is not happening, then this could be an option).
4. Best is sit in a quiet dark room by urself with ur eyes closed, focus on ur throat and chant aloud in ur favorite tune for 20 mins. If u happen to forget and stop, then you will soon realize (after all you must be sitting in a dark room with ur eyes closed for a purpose...) and without getting aggitated about the lack of focus, simply pull ur mind back to the chant. Its ok if while chanting aloud ur mind is running a parallel series of thoughts, that's how it normally happens for most people. Over a period of time, maybe months, ur thoughts will become fewer and it will be a very relaxing experience.
5. The best part is that it takes just a few days of practice for the mantra to get ingrained in your mind. If u chant in an interesting tune, then it will keep popping up again and again in ur mind throughout the day, and you will find urself chanting automatically.
Thus it will become effortless.