If you chant mantras on mala then you must remember not to cross Meru of the mala. But you rather start chanting in backward direction.
For example…First Mantra you start in clockwise direction as marked Blue in image. For second chant one should start anti-clockwise (Green) aka from where you finished your 1st chant.
You should never cross SuMeru of the Mala. It is believed that all the energy gets generated during chant is stored on SuMeru point and crossing it considered inauspicious as well as other reasons as mentioned in below YouTube Video. Best mala to use for Mantra chanting is Shiv-Shakti bead mala OR Crystal/Tulsi/Rudraksh Beads Mala. I personally use Crystal Mala.
Hope below image highlights the chanting method. In image, 1st chant is marked in Blue colour and second is in Green. The yellow threads part or Tip of Mala is SuMeru point, which should not be crossed during chanting.
Watch below video (in Gujarati) to understand more about chanting with Mala…Purpose, benefits etc…