Progressing in the Pass is easy, keep merging items as usual! 
When you merge, Level 1 Mystery Pass items will randomly appear on your board. You can merge these items up to level 4, which grants the most Mystery Pass progress. To progress, double-tap a Mystery Pass item to collect it.

The progress bar fills as you keep collecting these items. There are rewards after completing each milestone, and even more, if you have the Super or Premium Pass. Make sure to get the most out of it!