To jump from 3.52 M33 to 6.60, you must first install the official Sony update. How To Update Your PSP TO 6.61 Official Firmware
Your old ms0:/ISO/ folder still works. However, your old 1.50 kernel homebrew (those apps with two folders %__SCE__ and __SCE__ ) will work on 6.60. psp 352 m33 upgrade to 660 portable
From 3.71, the path is much smoother. You could jump directly to 5.00 M33-6, but we will go to official 5.00 first for stability. To jump from 3
Recommended build (practical)