Use :: or REM

::   commenttttttttttt
REM  commenttttttttttt

BUT (as people noted):

• if they are not in the beginning of line, then add & character:
  your commands here & :: commenttttttttttt
• Inside nested parts (IF/ELSE, FOR loops, etc…) :: should be followed with normal line, otherwise it gives error (use REM there).
• :: may also fail within setlocal ENABLEDELAYEDEXPANSION