I suggest you learn the ashtanga pranayama sequence by slowly working your way through the sequence in small increments, the same way that you learn the ashtanga asana sequences. At first your practice will be short, only two to four minutes long, but you can extend the time by repeating what you already know until you feel ready to move forward in the sequence.
When learning the different techniques of the five pranayamas, I suggest that you alternate between using the instructed portions and the uninterrupted portions of this video series. Use the instructed portion when you want to familiarize yourself with the pranayama techniques such as bhastrika or when you want to receive extra instruction or tips on how to better approach the technique. Use the counted led portion when you want to be guided through the sequence without interruption or extra instructions.